The simple secret of presidential politics in this era, recognized by few politicians and even fewer voters, is this: As the South goes, so goes the nation.

In every presidential election since 1956, the candidate who has won the largest number of electoral votes in the 11 southern states has walked into the White House. That is true of no other region. The Midwest and the West voted for losers in 1960 and 1976; the Northeast, in 1968. But not since the Dwight D. Eisenhower-Adlai E. Stevenson contest in 1952 have Dixie voters ended up on the losing side.

If the South finds itself setting the fashion for the nation a century after the Civil War, its taste is fickle. No party has been able to win its support more than twice in succession, and with its restless spirit, the South has supplied more votes to third-party candidates than any other region.

That is just one reason both parties are focusing an increasing portion of their resources and efforts on gaining and holding the high ground in the South. Dixie political battles in the next 30 months will determine not only the leadership of a region whose economic and social challenges are growing as fast as its population, but also the likely direction of the nation and its government as well.

As previous articles in this series have reported, there are powerful cross-currents tugging southern politics in opposing directions. Republicans are winning in the metro-area bedroom communities and the interstate corridors where most of the southern growth has been channeled. But Democrats are strongly rooted in the rural counties and the increasingly black big cities. Where they can keep their biracial coalition intact -- and in many states it is fraying badly -- they can still prevail.

Clues to the future direction of the South are likely to come in three stages: This November's midterm elections will indicate whether Republicans can hold the gains they have made in the Reagan years, particularly the four southern Senate seats they won on his 1980 coattails and the eight House seats they picked up in 1984. The elections will also measure the Democrats' ability to withstand a major GOP effort to crack the near-solid Democratic ranks of governors. Stage two will focus on the new southern super-primary in March 1988, when at least seven and perhaps as many as 10 of the Dixie states will pick national convention delegates in the same week. That will be as much of a contest for the party loyalties of southern voters (most of whom are free to pick between the Democratic and Republican primaries) as it will be a battle between presidential rivals. Finally, in November 1988, the South will declare its presidential allegiance for the post-Reagan era, and, not so incidentally, elect five senators and a host of state and local officials. When those returns are in, it may be possible to say who has won the heart of Dixie.

It is a crucial question, because the former Confederate states will have even more leverage on the nation in the 1990s. Projections indicate that the 11 states from Virginia west to Texas will gain 10 House seats and electoral votes after the 1990 census, giving them 126 of the 218 seats needed for a simple majority in the House and 148 of the 270 electoral votes necessary to win the presidency.

Even in this year's midterm elections, the southern prize is large. Races for seven Senate seats -- four held by the Republicans -- and seven governorships, all but one now in Democratic hands, make the region a prime focus. In 1984, nine of the 22 House seats which changed hands were in the southern states. Many of them will be battlegrounds again.

The Republicans have done well in southern states since Ronald Reagan's first run for the White House in 1980, gaining four Senate seats, 13 House seats and hundreds of other offices down to the courthouse level. As a result, Republicans counted on Reagan's momentum to carry them further at least as long as he occupies the White House. But that assumption is increasingly in doubt.

In east Texas last August, Republicans went all-out to win a rural House seat left vacant when Reagan appointed conservative Democratic Rep. Sam B. Hall Jr. to a federal judgeship. But Democrat Jim Chapman won the special election by 1,924 votes and this year has no Republican opponent.

Last fall, Reagan crossed the Potomac into Virginia, which had given him 62 percent of the vote, to campaign for the state GOP ticket. But Democrats swept the three top statewide offices by wide margins. After Virginia, Republican National Committee Chairman Frank J. Fahrenkopf Jr. warned his party that Democrats were moderating -- or appearing to moderate -- their stances and would be tough to defeat in other Dixie races.

This year's Senate races pose more risks to the GOP than to the Democrats. Of the four Republican-held seats, only Sen. Mack Mattingly (Ga.) is rated clearly ahead today, in part because the leading Democratic contender, Rep. Wyche Fowler Jr. of Atlanta, has the most liberal voting record in the Georgia delegation. There is less ideological distance between the likely opponents in Florida, North Carolina and Alabama, where well-known current or former Democratic officeholders are running. The Democrats are favored in Florida and considered competitive in the other two states.

By contrast, the only two southern Democratic senators seeking reelection this year, in Arkansas and South Carolina, escaped challenges from well-known Republicans. The main Senate bright spot for Republicans in the South is Louisiana, where Rep. W. Henson Moore (R) has been leading Rep. John B. Breaux (D) for the seat of retiring Sen. Russell B. Long (D).

Lee Atwater, a political consultant who played a key role in Reagan's two campaigns in the South, said, "If we have major losses in the southern Senate races in 1986, it will be hard for us to maintain there's been any realignment. Obviously, those seats are important to control of the Senate; but they're even more important to the future of the South."

The opportunity for GOP gains can be found in the state battles, especially the gubernatorial races. State government is increasingly important in the South, as elsewhere, as governors and legislatures struggle with challenges in education, economic development and environmental protection that will determine their states' futures.

The Republicans are only marginally involved in that effort today. Even after the gains of 1984, they hold only one of every five legislative seats in the South. Except for the two border-state GOP executives, James G. Martin of North Carolina and Lamar Alexander in Tennessee, the southern governors are all Democrats.

Atwater called the governorships "the most important missing piece in the realignment puzzle for Republicans," and Democratic consultant William Hamilton, who is involved in several Senate and gubernatorial contests in the region, said he agreed. "The governor's office gives Democrats access to the business community," he said, and "without that financial base, we lose one of our vital advantages in the South. I won't say losing southern governorships would be another nail in the coffin, but each loss would surely be felt."

Republican gains in the legislatures this year are likely to be modest at best, and in such states as Georgia and North Carolina, Democrats are seeking to erase beachheads Republicans won on Reagan's coattails in 1984. But there is a major -- if uneven -- Republican thrust on the governorships. Democrats are strongly favored in Arkansas and virtually unchallenged in Alabama and Georgia. But in Florida and South Carolina, where popular, two-term Democratic incumbents are stepping down, Republicans are bidding strongly to win governorships they have held only once since Reconstruction. That is true in Texas as well, where ex-governor William P. Clements (R) is in a rematch with Gov. Mark White (D), the man who ended Clements' breakthrough after one term in 1982.

If Republicans can annex any or all of those states, while holding Tennessee (where ex-governor Winfield Dunn (R) is a slight favorite to succeed the retiring Alexander), it would give them a real presence in the South. Earl Black, an expert on southern politics who teaches at the University of South Carolina, said those open governorships are probably the most important races this year in determining the South's future. "In the Reagan era," he said, "the momentum is plainly with the GOP, but if the Democrats can hold their governorships, they may stem the tide."

The problem, as Black pointed out, is that Democratic gubernatorial primaries -- such as those being fought in Alabama, Florida, South Carolina and Tennessee -- tend to strain a coalition already weakened by black-white divisions. In some cases, as in South Carolina, Republicans will be ready with a well-financed and well-tested candidate to pick up the pieces, and in others, such as Florida, they have a tough nomination contest of their own.

Beyond this year, 1988 provides a double test for both parties, first in the southern-dominated "super-primary" and then in the general election. Under the prodding of Democratic legislative and party leaders, seven states have joined the trio of Alabama, Florida and Georgia, which already held their presidential primaries on the second Tuesday in March.

In the South, Mississippi and Tennessee and border states Kentucky, Missouri and Oklahoma have shifted to primaries on that date. Virginia has decided to move its caucuses to Saturday of the same week, the same date the South Carolina Democrats have chosen. (The Republican date is not settled.) Legislation moving the Maryland primary date to mid-March is awaiting the governor's signature.

Arkansas, North Carolina and Texas are expected to consider a similar shift in their next legislative sessions and there is strong support for the move in these states as well. The prospect is that there will be a formidable bloc of votes -- close to 60 percent of those needed for nomination, based on the last delegate apportionment -- in the southern and border states that week.

There are two questions about the effect of the southern super-primary: its impact on the expected nomination fight in each party and its implications for the relative strength of the two parties in the South.

The state Democratic leaders who have pushed it have done so in hopes it will enhance the nomination chances of a moderate-conservative and perhaps a southerner, someone who could carry their states in November 1988 or at least not drag down the state and local tickets. Many northern Democrats, including party chairman Paul G. Kirk Jr., surveying the recent failures of nominees from north of the Mason-Dixon line, have agreed that the Democratic ticket needs a southerner on it in 1988. But there is great debate whether the super-primary will have the desired effect.

Some say it is made to order for a Charles S. Robb, the former governor of Virginia, or a Sen. Sam Nunn (D-Ga.). But so far neither has mounted a preparatory presidential effort. Others think that whoever wins a plurality victory in New Hampshire, however liberal he may be, will ride an unstoppable wave of publicity into the southern primary two weeks later. Still others think that if four or five credible white candidates are competing in the South, the Rev. Jesse L. Jackson can be the ultimate beneficiary of this new wrinkle in the nominating system.

Similarly on the Republican side, even though Vice President Bush appears now to have the backing of most party leaders across the South, the prospect of a Dixie super-primary has been welcomed by everyone from conservative evangelist Marion G. (Pat) Robertson to moderate former Senate majority leader Howard H. Baker Jr. (Tenn.).

Equally important -- though less discussed -- is the impact the southern primary may have on the Republicans' efforts to gain long-term adherence of southern voters. A central part of that drive is to convince southerners that they won't miss out on most of the action if they skip the Democratic primary. In 1988, thanks to the super-primary, Republican parties across the South will offer moderates and conservatives the chance to help pick Reagan's successor, at least as the Republican nominee and perhaps as president.

In eight southern states -- Alabama, Arkansas, Georgia, Mississippi, South Carolina, Tennessee, Texas and Virginia -- there is no party registration to serve as a barrier. Perhaps most southerners will "stay home" in the party of their parents and vote for a regional or ideological soul-mate in the Democratic primary, as the promoters of the super-primary hope. But if they find the Democratic choices annointed by New Hampshire less than appealing, there is a chance of a mass migration to the Republican primary that could significantly and permanently affect those voters' party identification.

Ultimately, of course, the test will occur in November 1988, when southerners will look at the nominees of the two parties and decide which comes closest to their ideal. Only Virginia among the southern states has voted the same way as many as three successive times in the presidential elections between 1968 and 1984. Having gone nearly unanimously for Reagan in 1980 and down the line for him in 1984, the Dixie states may be ready to bolt. If the South really gets to pick the 1988 Democratic nominee, and if that person is a southerner, the history of the last two decades say the odds are 2 to 1 that Democratic nominee will be elected.

But if Republicans sweep the South again in 1988 and once again broaden their base in state and local offices, the Democrats could find themselves facing real two-party competition such as the region has not known since the Civil War.

Either way, the South seems destined to put its stamp on the nation.