A paradox confronts both political parties at the start of the 1986 campaign year: Rarely have the tactics and issues seemed more parochial, but rarely have the stakes been bigger.

How big? Well, history says that the 1986 elections could write a premature epitaph for the Reagan era in American politics. But few Democrats are ready to bet that will be the case.

The recent trend lines say that 1986 could move the Republicans to within striking range of becoming the majority party, but no one at the White House is counting those realignment chickens as if they had hatched.

Pollsters and political strategists on both sides say 1986 will be a typical off-year election year in its emphasis on individual candidates and concerns in particular states and districts. But the consequences will be far greater than usual, as both parties position themselves for control of the post-Reagan era some say already has begun.

That long-term struggle overshadows the immediate battle for control of the Senate, where 34 seats are up, for 36 governorships, and for thousands of seats in the House and state legislatures across the land. When those results are in next Nov. 4, there will certainly be more evidence -- and there may even be a consensus answer -- to the question that has divided politicians and political analysts since the 1984 election: realignment or dealignment?

"Realignment is a myth, and 1986 will bury it," Democratic National Committee Chairman Paul G. Kirk Jr. said.

He and other dealignment advocates say President Reagan has helped the Republicans overcome their 50-year status as the minority party in the American electorate, only to find that partisan ties no longer mean much to voters. They see a free-floating electorate, picking candidates without regard to labels, largely on the basis of their records, their images and their television ads.

The dealignment camp predicts that there will be little in the way of a national pattern to the voting, that most incumbents of both parties will do well, and that little partisan advantage will be recorded for either side.

"This is a year when all politics is local," Greg Schneiders, a Democratic campaign consultant, said. "There are no crises or great divides. There's no intensity to the partisanship. People feel very free to vote for the candidate they like. It's strictly best-man-wins. Both parties are likely to come out alive and well and ready for 1988," the next presidential election year.

On the opposite side of the argument, supporters of the realignment theory say the Republican tide has been rising since 1978. They predict, against the odds, that it will rise again this year. They see an electorate -- particularly younger voters -- increasingly ready to pledge allegiance to the Republicans as the party of growth, opportunity and personal independence, and they expect further inroads on areas of traditional Democratic strength. They expect Republicans to hold onto the Senate, pick up some governorships, and make gains in the legislatures and among key voting blocs.

Where Democratic Chairman Kirk predicted that his party "will win back the Senate, add strength in the House and do very well in governorships," Republican National Committee Chairman Frank J. Fahrenkopf Jr. said, "I think we'll turn this 'six-year itch' upside down."

Typically, the party that controls the White House loses heavily in the midterm election of its sixth year in power. Mitchell E. Daniels Jr., the assistant to the president for political affairs, has pointed to the average loss of six governorships, seven Senate seats and 48 House seats in the "sixth-year" elections of the postwar period.

But 1986 does not look like a typical year. Severe losses in the past -- for Republicans in 1958 and 1974, for the Democrats in 1966 -- have been linked to a recession and/or a slump in the president's popularity. Reagan enters 1986 at historically high levels of support (66 percent in the latest Washington Post-ABC News poll) and the economy is finishing its third year of unbroken growth, with most forecasts that it will hold up at least until 1987.

That kind of year normally spells success for incumbents, and many observers foresee little shift in Senate and House seats, with greater volatility in governorships only because of the larger proportion of vacant seats. They note that the parties split the only two governorships at stake in 1985, with a Republican reelected in New Jersey and the Democrats retaining Virginia. Most incumbent mayors breezed to reelection last year.

They also foresee races where the personal, financial and organizational strength of individual candidates will be more important than any national trends or issues. While Republicans will enjoy a substantial overall advantage in party-generated funds, Democratic congressional incumbents do well with their money-raising from political action committees.

And the Democrats believe that they have put their stamp on issues that will help them in particular districts and states: relief for farmers; protection against foreign imports of shoes, textiles and other products; guarantees of Social Security and Medicare benefits. Even the president's pet domestic project, tax revision, bears a Democratic label after its passage by the House.

Citing these issues along with sanctions on South Africa and the fight against "defense waste," Rep. Tony Coelho (D-Calif.), chairman of the Democratic Congressional Campaign Committee, claimed that "Democrats won the major legislative and political fights of 1985" and are now in a stronger position for 1986.

But as the year begins, there is at least a possibility that some of the deeper shifts that were seen in the 1980-84 elections may manifest themselves again. If they do, the Republicans are almost certain to be the beneficiaries, for those elections saw the breakup of the old Democratic coalition and the emergence of a new pro-GOP majority in both presidential and Senate voting.

That possibility is what makes presidential pollster Richard B. Wirthlin say that "for me, the critical election of the decade is 1986. We have to hold the Senate and see gains in the states and in voter registration figures, or the 1980-84 years will reflect only a move toward a personable and attractive president.

"But if we see young people staying the most Republican group in the electorate, if we see first voters registering Republican . . . , then we'll have a good chance to win the presidency again in 1988 and become the majority party of the 1990s."

Even some Republicans find Wirthlin's scenario overly optimistic. But it cannot be dismissed. Recent polls show the Republicans maintaining their edge as the party most likely to bring peace, prosperity and a better future. This appeal is particularly striking among young voters, who cast their first or second presidential votes for Reagan in 1984 and who may or may not show up for the less glamorous congressional and state contests of this year.

Republican pollster Robert Teeter conceded that the GOP hold on those and other independent-minded voters who supported Reagan "is fragile and volatile. But the longer we can keep the coalition together, the more solid it becomes . . . . We survived 1982 and did very well in 1984. We don't need to do spectacularly in 1986, but if we hold on reasonably well and elect a president in 1988, we'll be pretty solid."

When the politicians say that the stakes are unusually high in 1986, here are some of the questions they think this election will answer: Control of the Senate

Daniels said this is the White House's top priority and may be the Republicans' toughest challenge. Analyst Kevin Phillips has pointed out that, since 1950, in the five midterm elections held when they controlled the White House, Republicans have usually managed to win fewer than one-third of the Senate seats at stake. Their best showing, in 1982, was 39 percent. This year, with 22 Republican and 12 Democratic seats up, Republicans would have to win 65 percent of the races to maintain their current 53-47 majority, and 56 percent to eke out a 50-50 tie which Vice President Bush could break for the GOP in organizing the Senate.

That sounds like a large order, and it is. But it is vital to Reagan and to the GOP future. Without leverage in either chamber of Congress, Wirthlin said, "the president will be very much on the defensive in 1987-88 and he doesn't play well on defense."

Regaining control of the Senate "would be a tremendous psychological lift to our party," Democratic pollster Paul Maslin said. With control, Democrats could use committee hearings and legislation to draw issues for 1988 on everything from Reagan's judicial appointments to his budget priorities. On the other hand, Schneiders and some other Democratic strategists worry about what Reagan might do in making a Democratic-controlled Congress his whipping boy.

"I can't imagine anything more disastrous for the Democrats than to come out controlling the Senate by one vote," Schneiders said. "It would be a nightmare for the leadership, facing a popular president of the other party. We'd be liable for blame when things go wrong, and yet not really in control."

But others point out that Democrats cannot afford to fail in the Senate races this year, because the odds get steadily worse hereafter. In 1988, Democrats must defend more Senate seats than Republicans, and in 1990, they are at par. "If we don't get it back this year," said Peter D. Hart, a favorite pollster for Democratic candidates, "there will be more Russell Longs and Tom Eagletons." Sens. Long of Louisiana and Eagleton of Missouri are retiring this year.

Their seats are targets for the GOP, as those of retiring Sens. Charles McC. Mathias Jr. (R-Md.), Paul Laxalt (R-Nev.) and John P. East (R-N.C.) are for the Democrats. But the major battlegrounds will probably be the South and the Farm Belt, where 10 freshmen Republicans, elected on Reagan's coattails in 1980, will be facing the voters.

By contrast to the Senate, the House looks quiet this year, with most of the battles expected in open seats and general expectation of a modest Democratic gain. Control of State Government0.

While the battle for the Senate is sure to occupy the headlines, strategists in both parties consider the long-term stakes even greater in the 1986 gubernatorial battles. Democrats now control 34 of the 50 governorships and 64 of the 96 partisan legislative chambers, with two tied. That superiority gives them the upper hand in the arena of government that increasingly is taking the lead in domestic policy-making. It also gives them a head start toward controlling the redistricting of U.S. House and state legislative seats that will follow the 1990 census.

But 1986 is a year of vulnerability for the incumbent Democratic governors. Of the 36 governorships at stake, 27 are held by Democrats. Thirteen of the Democrats (and only four Republicans) are at the ends of their terms. The chairman of the Republican Governors Association, New Hampshire Gov. John H. Sununu, said the GOP hopes to make a net gain of at least six states, focusing on open governorships in states -- such as Arizona, New Mexico, Wyoming, Colorado, Oklahoma, Nebraska, Florida, South Carolina and Maine -- where Reagan ran strongly.

Encouraged by the takeover of the New Jersey assembly in 1985, Fahrenkopf said that for the long term, "the key for us is how we do in the legislatures. We're within 10 seats in 18 chambers now. If we can come out with a plus in a tough year like 1986, I'll be happy."

But Republicans must worry about holding governorships in three key states, California, Illinois and Pennsylvania, where Democrats are gearing for major challenges. A loss in any one of those states would erase the value of several small-state victories. And overall failure to dent the big Democratic statehouse majorities in 1986 would certainly send a message that the "Reagan revolution" is something the voters want confined to Washington, D.C. Political Future of the South

In 1984, Reagan's share of the southern white vote reached 71 percent and he carried counties that had been Democratic since Civil War days. Moreover, in some states, notably North Carolina and Texas, voters not only backed Republicans for the Senate and House in increasing numbers but also elected Republicans to courthouse offices for the first time.

As public affairs consultant Horace Busby, a Democrat, wrote recently, "The dramatic decrease in southern counties voting Democratic in 1984 means that 1980 may be considered as the last stand of a history long crucial to the party."

Republicans are pushing hard to make the breakthrough permanent. They have recruited former Democratic officeholders as candidates for statewide office this year in Texas, Florida and several other southern states. Dixie will be a special target, Fahrenkopf said, for Republican generic ads and registration efforts targeted both at new voters and at those who consider themselves Republicans now but retain their Democratic registrations.

Democrats are concerned. "The South is the area that has us the most worried," pollster Maslin said. "Clearly the party preference has moved dramatically." But Democrats were encouraged by their ability to hold a House seat in rural east Texas against a major GOP effort in a special election last summer and were even more heartened by their sweep in Virginia last November. Fahrenkopf has pointed to the tactics of the latter race as the model "the Democrats may use against us in 1986." The Democratic gubernatorial winner there received 48 percent of the white vote and heavy majorities from blacks, a coalition that can keep the Democrats in power almost anywhere.

The Texas governor's race and the Florida Senate race probably will be the most expensive and headlined battles in the region, but analysts will be looking further down the ballot to see if the GOP can keep and expand its foothold in the legislative and courthouse offices that supply an endless stream of candidates and campaign resources to the Dixie Democrats. Role of Key Voting Groups

After 1984's stunning results, in which voters under 30 supported Reagan over Democratic presidential nominee Walter F. Mondale by almost 20 percentage points, everyone has his eye on this key voting bloc.

Republican pollsters said, with fingers crossed, that they think the young voters may stick. "They're still fairly enamored of the Republicans today," Wirthlin said, citing even higher approval ratings than the older voters display for Reagan's handling of foreign policy and the economy.

Fahrenkopf cites polls showing the Republicans 7 points up on the Democrats in party identification among people ages 18 to 29, and even further ahead among their younger teen-age brothers and sisters. "It doesn't mean realignment is an accomplished fact," he said, "but we have the opportunity to become the majority party."

But there is conflicting evidence. In the 1985 Virginia gubernatorial race, the Democrats carried the younger voters. Lee Atwater, a Republican consultant who has focused on the "baby-boomers," cautioned that the GOP must make it clear it is "tolerant on the social issues" and "be careful of antigovernment rhetoric." The new voters, he said, "are 'pro-choice' on everything," and while they insist on "excellence and efficiency" from government, "they realize they need government in their lives."

Both parties will also be working to expand their support among women and minorities. Republicans have taken encouragement from New Jersey Gov. Thomas H. Kean's feat in gaining 60 percent of the black vote on the way to his 1985 reelection sweep, and are looking for inroads in other states, while continuing their avid courtship of Hispanic voters.

They have recruited Wayne County executive William Lucas, a black, and Tampa mayor Bob Martinez, a Hispanic, both former Democrats, as candidates for governor in Michigan and Florida respectively. But neither is given as good a chance of prevailing in both the primary and general election as Los Angeles Mayor Tom Bradley (D), a black, who is aiming for a rematch with California Gov. George Deukmejian (R).

The number of women candidates for Congress and statewide office is on the increase, but only three so far have surfaced as strong contenders. Lt. Gov. Harriet Woods (D) is the likely nominee of her party for the Missouri Senate seat that Eagleton is vacating, and former secretary of state Norma Paulus (R) has the same status for the GOP in the Oregon gubernatorial race. Early polls put Rep. Barbara A. Mikulski (D-Md.) out front for the Democratic nomination for the Maryland Senate seat that Mathias is vacating. Impact of Economic Issues

Pollsters in both parties find no overriding issue and no great interest in politics, at the national level. But trade issues are hot in textile, shoe and steel communities, especially after Reagan vetoed relief for the first two of those industries. Even after the president signed the December farm bill that broke his stated budget limits, Senate Majority Leader Robert J. Dole (R-Kan.) still fretted that the depressed agriculture sector "may cost us one or two" Senate seats.

A larger question mark is the federal deficit, which Wirthlin's partner, Vincent Breglio, said is "moving up very fast from a blip to be equal with unemployment" as an issue of concern to the public. Most voters blame Congress, rather than the president, for the problem, and Republicans congratulated themselves for dodging the bullet with last session's passage of the Gramm-Rudman-Hollings "automatic" deficit reduction plan.

But the first two installments of those cuts come due this year -- in March and October -- and they could be a time bomb for incumbents. Breglio told Republican governors last month that voters want deficits cut, but oppose trimming such expensive programs as Social Security, Medicare, farm and low-income aid and oppose a tax increase.

The president has put defense, Social Security and raising taxes off limits, and the resulting squeeze could pit Republican senators running for reelection against Reagan, in much the fashion that House Republicans found themselves at odds with him in December over the tax-revision bill. Such a split with the party's chief symbol and biggest campaign asset could cloud GOP prospects. 1988 Presidential Hopefuls

Dole and Rep. Jack Kemp (R-N.Y.) must face the voters this year before launching their expected challenges to Vice President Bush in the 1988 Republican presidential primaries, but neither faces major opposition.

On the Democratic side, assuming that Sen. Gary Hart (D-Colo.) announces this weekend that he will not seek reelection to the Senate, New York Gov. Mario M. Cuomo, Rep. Richard A. Gephardt (D-Mo.) and Sen. Dale Bumpers (D-Ark.) are the only speculative presidential contenders with a date at the polls this year. All of them are expected to roll up flattering margins.

Whether 1986 adds names to the prospective presidential list is uncertain, but observers will be watching the showing of such dark horses as California Gov. Deukmejian (R), Illinois Gov. James R. Thompson (R), Massachusetts Gov. Michael S. Dukakis (D), Arkansas Gov. Bill Clinton (D) and Texas Gov. Mark White (D), all seeking reelection this year, and Florida Gov. Robert Graham (D), who is running for the Senate.

They, or their conquerors, could gain consideration for the national ticket, as could such charismatic newcomers as Rep. John S. McCain III (R-Ariz.), favored to succeed retiring Sen. Barry M. Goldwater (R-Ariz.), and former secretary of transportation Neil Goldschmidt (D), running for governor of Oregon. Balance of Power in Parties

Even if the 1988 GOP and Democratic fields are not changed by the 1986 results, the internal configuration of both parties could shift.

Republican governors historically have been more progressive and less ideological than the party's congressmen, and an increase in gubernatorial ranks almost automatically translates into an infusion of strength for that underdog part of the party. Among the GOP moderates given good chances of winning governorships this year are Rep. John R. McKernan Jr. of Maine, ex-governor James A. Rhodes of Ohio, ex-governor and ex-senator Henry A. Bellmon of Oklahoma, Oregon ex-secretary of state Norma Paulus, Lt. Gov. William W. Scranton III of Pennsylvania, ex-governor Winfield Dunn of Tennessee and, in some reckonings, Rep. Carroll A. Campbell Jr. of South Carolina.

Teeter, a pollster with links to Bush and the moderate Republicans, said recently that, if Republicans cash in "on the opportunity we have to elect more governors in 1986, we can become what our counterpart in Canada is, the majority Progressive-Conservative Party."

On the other flank of the GOP, three of the Senate's more prominent conservative activists, Laxalt, Sen. Jesse Helms (R-N.C.) and Sen. Phil Gramm (R-Tex.), are sponsoring candidates for senator or governor in their states, and in the case of Helms and Gramm are being opposed by other factions of the GOP in the primaries. The outcome of those contests, too, will be watched for clues.

On the Democratic side, the effort to shift the party's focus away from traditional liberalism has been led by a band of governors and members of Congress from Virginia out to Arizona.

With such major "revisionist" figures as Arizona Gov. Bruce E. Babbitt, Colorado Gov. Richard D. Lamm and South Carolina Gov. Richard W. Riley among the eight southern and western Democratic governors retiring, the credibility of the movement depends in part on the Democrats' ability to keep control of those states -- as they did in Virginia last November, where outgoing Gov. Charles S. Robb (D) helped engineer a Democratic sweep -- and to elect such potential adherents as Los Angeles Mayor Bradley and ex-senator Adlai E. Stevenson III of Illinois, both running again for governor in their states.

Losses in those states, combined with reelection victories for such figures as Cuomo, Dukakis, Sen. Alan Cranston (D-Calif.), Sen. John Glenn (D-Ohio) and Wisconsin Gov. Anthony S. Earl (D) would surely suggest that liberalism is still alive in the fight for the future of the Democratic Party.