THE POLITICAL EXPERTS seem to agree that the Democrats will make modest gains in the Nov. 2 elections, but it's time everyone braced for a different possibility: a landslide giving Democrats up to 30 new House seats, plus substantial victories in Senate and gubernatorial races as well.

Indeed, if they play their cards right, the Democrats could win 40 or more House seats and have a genuine, if still remote, chance of regaining Senate control.

These no doubt will be seen as rash suggestions, or as mere wishful thinking by a Democratic pollster. What about the expected low voter turnout that traditionally favors the GOP? How about the massive Republican war chest? Or President Reagan's popularity? Where's the evidence suggesting that all those seats suddenly might head into the Democratic column in the campaign's last two weeks?

The evidence is there if you look at a pattern that's emerged in recent elections -- a pattern of stunning voter swings in the final weeks and even days, and predominantly against the party in power.

As for a low voter turnout, it could actually benefit the Democrats this time. Overflowing party coffers, while certainly helpful, guarantee nothing, as shown in numerous races, including the recent primaries. Democrats Ed Koch in New York and Ed King in Massachusetts -- both favorites of Ronald Reagan -- spent nearly three times more on television than their gubernatorial primary opponents, only to buy decisive defeats. They lost largely because their opponents hammered away at the Reagan program with which Koch and King were associated.

Which brings up the most important point: Ronald Reagan today is not nearly as popular as he's thought to be. The Gallup Poll, for example, shows not only that Reagan's mid-term rating on job performance is the lowest for any president in 30 years, but that his "personal popularity -- despite a widely held belief -- is not disproportionately greater than his predecessors' " at similar moments.

In short, with aggressive Democratic attacks, the Republicans could easily be overrun by a confluence of massive unemployment, anxiety over cuts in Social Security and other social programs, anger at tax breaks for the rich, near-Depression conditions in the industrial and farm belts, hostility to Reagan from women, minorities, environmentalists and the poor everywhere, concern about nuclear arms and much else.

Unfortunately for the president and the GOP, elections -- particularly mid-term elections -- are chiefly a reflection of current conditions rather than a referendum on past blame or future expectations: "What have you done for me (or to me) lately?" is the operative battle cry.

The Democrats themselves, of course, are the ones who must close the gap between a probable victory and a potential landslide. Too often they have been sluggish and tentative in arguing their case against the GOP. Too often Reagan's supposed popularity has rattled them and made them defensive, leaving the impression that they are uncertain of what they stand for -- and, more importantly, what they stand against. Too often they seem to forget that an election is a forced choice between imperfect alternatives and that when your case is politically stronger, you don't spend your time parrying your opponent's solid but weaker arguments.

In fact, one must admire the pugnacity of the president and his party. Outnumbered and out-positioned, they nonetheless are always on the attack. For months they have forced battles on favorable grounds, often by mastering the arts of disinformation (a la their notorious Social Security ad) and bravado. As Republican strategist John Sears has noted, with unemployment at a 42-year record of 10.1 percent, the Democrats deserve a dunce cap for being drawn into discussions of tax cuts versus balanced budgets; they should have been arguing about the plight of working people.

If the Democrats resist their instinct for the capillary and strike for the jugular, they will be remarkably successful even if their message is unembroidered by "positive" solutions. Off-year elections do not demand positive programs from the party out of power; simply being the alternative usually suffices, particularly in election years as troubled as this one.

In 1980 the political agenda was virtually a Republican manifesto: inflation, government spending, high taxes, defense budgets, American weakness and scores of social issues. In 1982 the agenda of concern has moved to Democratic high ground: long unemployment lines, Social Security, fairness, slashed social programs, peace, and environmental protection (plus normally GOP worries, such as massive budget deficits and business bankruptcies, that have worsened under Reagan). Couple this issue shift with the normal off-year bias against the incumbent party and the Democratic House gains already reaped from redistricting, and the potential for a landslide becomes evident.

That only two weeks are left in the campaign doesn't change this prospect, particularly in light of recent election patterns.

Before 1978, off-year elections were pretty well set by the third week of October; those upsets that did occur generally involved races that were already relatively close. But the campaigns of 1978, 1980 and 1981 brought a striking phenomenon: violent, last-minute shifts in voter sentiment -- to an unprecedented degree against the incumbent party -- that turned once-stable general elections into races akin to volatile primaries.

In 1978, politican consultants, reporters and pollsters -- including this one -- were caught napping by GOP upsets over relatively popular incumbent senators. In every case the GOP candidates had trailed by large margins as late as Oct. 20, suggesting to those familiar with election history that such races were over. They were not. Many of the upsets were characterized by late media blitzes, using almost exclusively negative campaigns.

The phenomonon reappeared in 1980. At the presidential level, the last week of the campaign saw the race go from a virtual tie on Saturday to a 5-point Reagan lead on Sunday to a 10-point lead on Monday, election eve. Voter preferences for Congress, despite five months of relatively stability, also broke toward the Republicans in the closing hours.

Just last year in Virginia and New Jersey, similar late breaks were observed, in these instances toward the Democratic gubernatorial candidates. And, although they are different from general elections, one sees the trend yet again in this year's primaries.

Two weeks before the New York primary, one in which I was involved, Mario Cuomo trailed Ed Koch by 17 points. At that time a determined Cuomo campaign of speeches, debate appearances and paid media kicked into high gear, heaving anti-Reagonomics themes against Reagan's favorite mayor. Cuomo won his upset by 6 points -- a turnaround of more than 23 points in the final two weeks.

The late-break trend is just one factor that is currently being underestimated. The degree of economic distress and the anxiety it is producing is another.

As any survey analyst worth his salt can tell you, the voting impact of unemployment is considerably greater than the voting impact of inflation. That becomes crucial when one recalls estimates suggesting that 15 to 25 percent of the work force has been unemployed at some point over the year, making the jobless experience -- to say little of the fear -- more widespread than commonly understood.

Republican analysts are correct in saying that the joblessness factor is somewhat restrained by an abiding concern over inflation. Also, a number of voters have yet to render a final verdict on Reaganomics, and many retain a hope that it will yet work. These straws have been clutched by the president and his party about as well as anyone could imagine.

But Reagan will likely find a public unwilling to believe, ab much less applaud, the GOP's "good news" about inflation and the stock market. In the end elections are forced choices, demanding a resolution of competing instincts. They are not essays of "On the one hand this, on the other hand that."

The Republican theme of "Stay the Course" might have some impact if the election were truly a contest between "Stay the Course" and "Go Back to . . ." But in an off- year election, the more likely choice in voters' minds is between "Stay the course" and "Let's Make Some Changes." In that referendum, the polls show the president losing by about 2 to 1.

A third underestimated factor is the potential nature of this year's voter turnout.

Normally, a low turnout favors Republicans, and many experts are predicting an even lower turnout in 1982 than in 1978, which was the lowest since the war year of 1942. But the conventional wisdom may not apply this time.

What is important in a low-turnout election is who turns out. The Massachusetts and New York primaries -- where anti-Reagan- type voters turned out in far greater numbers than those who generally support Reagan's policies -- suggest that a similar pattern may emerge elsewhere. If so, the damage to the GOP would be severe.

Remember that a number of voter groups, particularly minorities, women's rights activists and environmentalists, have felt the sting of Reagan policies for almost two years. They not only have little enthusiasm for "staying the course" -- they have a real interest in "getting even." Nothing moves voters like the prospect of retribution.

On the other side, moreover, are the cross- pressured Republican voters. It is not hard to imagine a normally Republican farmer who is being devastated by current policies resolving his mixed emotions by staying home Nov. 2. In sufficient numbers, in key areas, such nonvoting could be disastrous for the GOP.

A fourth underestimated factor: The role of the parties could be unusually detrimental to the GOP. In the face of declining party allegiance, the Republicans in 1980 boldly offered a party alternative to the country that has fashioned a new role for the parties. It has caused Democrats of all stripes to pull together. If the people have not become more party-line voters, then at the least the parties have come to serve as standards for issue agendas around which voters can assemble. This makes it easier for voters to put aside individual candidate's own records and personalities.

Indeed, the president has made this prospect even more likely through his determination to nationalize the election with his "Stay the Course" speeches -- and on the GOP's weakest ground, the economy.

The significance of this party development will likely be noticed in Republican incumbents' congressional districts, and particularly in Senate races of popular Republicans. In recent years, the Senate is where voter frustration seemed generally to have been channeled against incumbents. If that trend continues, Republican senators who enjoy personal popularity may find themselves facing stiffer challenges and potential defeat as many voters attach themselves again to party labels on the economic terms the president has been setting.

Next, the money factor. There's no doubt that the war chest is the GOP's single greatest weapon, but two factors serve to cushion its impact.

First, a number of recent elections have demonstrated that a candidate funded at a level adequate for his or her message to get across -- or to reach a threshold of public awareness -- can prevail. Second, if a candidate is riding a tide of the right issue, themes and positions, that tide can wash over a seawall of money.

Finally, but perhaps most important, there is misunderstanding about Ronald Reagan himself. Even today one hears the universal acknowledgement of the president's "popularity," an assertion with an uncanny resemblance to the emperor-with-no-clothes tale. Perhaps no president in the modern era was elected with so many negatives, so many doubtsab, so many concerns as was Reagan in 1980. In his first year, Reagan won a string of impressive political victories while simultaneously accumulating the highest negative ratings of any first-year president. Today his job ratings not only have remained lower than his predecessors', but he has become the first president of the modern political era to fall behind well-known opposition candidates (Kennedy and Mondale) in the Gallup pairings during his second year in office. Presidents Johnson, Nixon, and Carter did not fall behind until midway through their third years, and then usually only for a short duration. (For some reason most political observers mentally keep comparing Reagan's mediocre second-year popularity with the last year unpopularity of his crippled predecessors, Johnson, Nixon and Carter.)

The experts have been quick to reply, "Well, Reagan remains personally popular." To an extent that is true, just as it was with Ford and Carter before him. But while polls have shown he is perceived as likeable and genial, they also say that he is uncaring to an unprecendented degree -- the very stuff of real political popularity. While viewed as strong and decisive -- important presidential characteristics -- Reagan is also suspect for being dogmatic and stubborn.

The different between perceptions and realities reagarding Reagan is perhaps best explained by the class bias of Reagan's constituency. The opinion makers usually are in the higher income ranges, living daily among the group that gives Reagan his best ratings. They cannot see around them how sharply esteem for Reagan and his policies drops with middle- and lower-income people -- but there is no doubt that it does.

Although such a view is political heresy, particularly in Washington, the truth of the numbers remain. And those who keep insisting the contrary should be compelled to produce evidence for their viewpoint. Reagan's ratings are rather ordinary, even mediocre, in a genuine, historically comparative sense. One finds this true not only in national polls but, with a few exceptions, in state poll after state poll.

In every model of off-year election analysis, presidential job approval, together with state of the economy, is a leading influence on voting. That is why the Democrats should not hesitate to engage not only the administration or the GOP but the president himself on issues of substance. (This doesn't mean they have license or reason to savage the president personally; history suggests that that is always a bad tactic.) In particular, the president's highly charged, partisan attacks and assertions, which bear at best a tenuous and slender relationship to truth, invite a full and direct response by Democrats.

Reagan has especially exposed his flank to Democratic attack by appearing unreasonably dogmatic and rigid on the economy. He is dangerously signaling the public that a GOP victory would be a mandate to resist any change in economic policy. Such a posture is frightening not merely to liberal Democrats but to many who generally support the president's direction.

If the Democrats hammer away for two weeks at these Reagan vulnerabilities, they might well wake up on Nov. 3 to discover that they have won a landslide far greater than they dare predict today.