Regardless of who wins the presidency Tuesday Congress will almost certainly be more Republican and more conservative when it reconvenes in January, although the Democrats are not expected to lose their quarter-century lease on the congressional leadership roost.

The outcome in many tight races may hinge on forces beyond the candidates' control, including the latest developments in Iran and their impact on turnout and voting preference in the presidential race.

As of last week, campaign strategists from both parties projected a Republican gain of 15 to 25 seats in the House. There were so many close races for the Senate that the Republicans could pick up a half dozen or more seats -- or none at all.

In any case, with the Democrats now holding 273 of 435 House seats and 59 of 100 Senate seats, the Republicans see no change of capturing the House and only a longshot hope of claiming the Senate. It would take 10 added seats, or nine seats and a Republican vice president, to turn the Senate around.

But even modest gains could position the GOP for an all-out drive for control of Capitol Hill in two years, which has been the party's goal all along.

Moreover, numbers alone do not tell the whole story. Many of the vulnerable Democrats are key committee chairmen and prominent leaders of the party's long-dominant liberal wing. And the party-line margins tend to obscure a closer ideological alignment in the two houses. Thus the loss of a relatively few critical seats could have impact beyond their numbers.

Consider, for instance, the list of most-endangered Senate Democrats that a Democratic campaign strategist came up with the other day.

It reads like a cross-sectional sampling of the party's liberal past, present and future: George McGovern (S.D.), Frank Church (Idaho), Birch Bayh (Ind.) and Gary Hart (Colo.). McGovern was the party's 1972 presidential nominee. Church, chairman of the Foreign Relations Committee, and Bayh, most recently chairman of the subcommittee that investigated Billy Carter, were candidates for the Democratic presidential nomination in 1976. Hart is regarded by some as a possible future presidential contender.

A preelection poll in the roller-coaster Iowa Senate race also puts Sen. John C. Culver (D), who wore his liberalism like a badge of courage during the campaign, back in a prominent position on the endangered list.

In addition, Senate Appropriations Committee Chairman Warren G. Magnuson (D-Wash.) and Senate Agriculture Committee Chairman Herman E. Talmadge (D-Ga.) have run scared, as have Sens. John Durkin (D-N.H.), Gaylord Nelson (D-Wis.) and Patrick J. Leahy (D-Vt.), although all but Durkin are now believed to have an edge. Close fights are being waged for the seats of Sens. Richard Stone (D-Fla.), Mike Gravel (D-Alaska) and Donald W. Stewart (D-Ala.), who were defeated in primary elections earlier in the year.

At the same time, however, the seat now held by Sen. Jacob K. Javits (R-N.Y.) could fall to the Democrats. Races for the seats being vacated by Sens. Henry Bellmon (R-Okla.) and Richard S. Schweider (R-Pa.) are closer, with the Republicans now favored in Pennsylvania. Unseating Sen. Barry Goldwater (R-Ariz.) is still a distant Democratic hope.

In the House, the longer list of shaky Democratic seats includes those held by Majority Whip John Brademas (D-Ind.), Ways and Means Committee Chairman Al Ullman (D-Ore.), Interior Committee Chairman Morris K. Udall (D-Ariz.), Agriculture Committee Chairman Tom Foley (D-Wash.), Democratic Campaign Committee Chairman James Corman (D-Calif.) and Public Works Committee Chairman Harold (Bizz) Johnson (D-Calif.) -- although Ullman, Udall and Foley now appear to be leading. So does House Majority Leader Jim Wright (D-Tex.), who was confronted with an unusually tough race. Several Democratic Abscam defendants are in trouble, too, although the Democrats could pick up the seat now held by Abscam defendant Richard Kelly (R-Fla.)

The Republicans started the campaign with big presumed advantages, including bulging campaign coffers, a sophisticated strategy for recruiting and running candidates, and the widely touted liabilities of incumbency -- especially Democratic incumbency -- in 1980. Moreover, the Democrats were defending a lot of marginal seats, many dating back to their 1974 post-Watergate sweep. In the Senate they were defending 24 seats, while the Republicans had to protect only 10.

But the early warning signs of Democratic vulnerability may have saved the Democrats from worse troubles than they face now, forcing even the most elite of the congressional elite to show up at factory gates, shopping centers and senior citizen centers and learn again what it's like to grub for votes. Many voters may already have gotten their pound of flesh -- in the form of inches off the portly frames of their elected representatives.

Examples abound in both the Senate and House races.

Both House Ways and Means Chairman Ullman and Senate Appropriations Chairman Magnuson have been campaigning as rarely before since disappointing showings in their party primaries, coupled with other warning signs, signaled trouble. Both new enjoy at least small preelection advantages.

Senate Agriculture Committee Chairman Talmadge had to learn the lesson twice. After he failed to win a majority in the Democratic primary and had to slog through a hard-fought runoff, Talmadge coasted on the assumption that he had no Republican problems -- only to come hustling back to Georgia for yet another defense of the homestead.

The Senate race in Iowa may best dramatize the whipsawing of Democratic incumbents, despite their best efforts to combat it.

Two years ago, a right-wing effort caught then-Sen. Dick Clark (D-Iowa) by surprise and defeated him. Forewarned, Culver began early to mend fences at home. Despite ominous polls that had him running as much as 17 percentage points behind popular conservative Rep. Charles E. Grassley (R-Iowa) only a few months ago. Culver ran flat-out on his largely liberal record, and gradually forced Grassley onto the defensive. By early October, the Iowa Poll showed Culver snatching a lead; a late October version of the same poll showed Grassley snatching it back.

Another hallmark of the 1980 campaigns was the extent and intensity of right-wing independent efforts waged by the well-financed National Conservative Political Action Committee, antiabortionists and other groups against Culver and other liberal incumbents. There are some signs of a blacklash, but how much is unclear.

Yet another factor in the final shapeup of the congressional races was the purifying effect of Democratic primaries that replaced vulnerable incumbents with fresher-faced contenders. In Alaska, for instance, it can be argued that the Democrats' chances are at least as good without Gravel on the ballot.

In the case of New York, the Republican primary defeat of Javits by archconservative Alfonse D'Amato opened the way for a strong bid by Rep. Elizabeth Holtzman (D), even though Javits remains in the race as the Liberal Party candidate and the Holtzman-D'Amato confrontation has tightened.