Not since the poltical aftershocks of Watergate almost blew the Republican Party out of Congress six years ago has the GOP seen such a comeback opportunity on Capitol Hill.

Even in the view of Republican strategists, the chances of Congress next year for the first time in a quarter century are remote -- slight in the Senate and almost nonexistent in the House.

But if the Republicans can translate their theoretical opportunity into votes this November, they could set the stage for a full-fledged battle for control of the nation's legislature in 1982.

Whipsawed by forces ranging from Iran to the Consumer Price Index, the Democrats are divided and defensive, forced to defend some of their shakiest congressional perimeters at a time of high peril for incumbents.

A half dozen or more liberal Democratic senators, who profited from the crumbling of a Republican administration when they last ran in 1974 but must now run a tandem with a troubled standard-bearer of their own party, are viewed as potential casualties. Abscam has jeopardized a few Democratic House members who might otherwise be strong bets for reelection.

Moreover, threaded through the 435 House contests and 34 races for the Senate are enough domestic and foreign policy problems to make any incumbent squirm. And most of the incumbents whose terms expire this year are Democrats: 274 in the House and 24 in the Senate.

"It's a tough year," said Democratic pollster Peter Hart. "We're not exactly jumping for joy around here," added an aide to the Democratic Senatorial Campaign Committee.

At the same time, Republicans are luring out some well-credentialed candidates who thought better of running under the GOP flag in the 1970s, and some polls indicate that voters are beginning to shake off their low opinion of the GOP's capacity to deliver on their major concerns, especially the economy.

Although still a meager minority in the House, the Republicans are sitting on top of victories in seven of 10 special House elections held since 1978, an impressive launching pad for 1980.

In light of all this, it may seem curious that, as of now, with all due regard to the volatility of politics this year, the odds-makers favor at least another two years of Democratic dominance on Capitol Hill.

Senate Majority Leader Robert C. Byrd (D-W. Va.) admitted in his weekly news conference yesterday that conditions these days are "not ideal" for Democratic officeholders and that "it is conceivable" the party could lose its majority in the Senate.

"I'm not predicting such a thing, but it has happened before," Byrd said. He added he is concerned about the prospects of the Democrats, who have more than twice as many seats at stake as the Republicans.

"I'm hopeful that the economic situation, the energy situation and the international conditions will change by [election day]," he said. "That's the best politicking of all.

The Republicans, normally not very bashful about their comeback predictions, are confining themselves to claiming only a big enough boost to get them to the gate for 1982: 15 to 20 seats in the House and three to six seats in the Senate.

Republican National Chairman Bill Brock states the odds this way: "Even money that we'll gain enough in both houses to have an excellent chance of getting control in 1982." Even more cautiously, Steven Stockmeyer, executive director of the Republican Congressional Committee, says. "It's entirely feasible that 1980 could set the stage for a very competitive battle for control in 1982."

A look at the numbers -- and the recent history -- helps explain the caution.

In the House, Republicans have managed to calw their way back to 159 of 435 seats after the post-Watergate debacle six years ago when they plunged to 144 seats. Even if they gain 20 seats, as they hope, they would still be short of the 187 seats they had before Watergate. It would take 218 to get control.

One of the surprises of recent congressional politics has been the staying power of the huge Democratic Class of '74 in the House. In its hands, constituent service became a political art form. "In some cases," said Stockmeyer, not without a flicker of envy, "people knew they had a congressman for the first time because they got a newsletter six times a year," Republican gains in the last few years have come largely from outside the ranks of Class of '74, he noted.

One bright spot for the Republicans this year, Stockmeyer said, is that concern over constituent services is beginning to lose ground to concern over issues in voter surveys -- and the Republicans are doing better on issues.

"It's the first election in my lifetime that we've been favored on economic issues," said Brock, referring to GOP and other surveys that show the Democrats losing their edge over Republicans as more able to cope with inflation, taxes and budgets.

The Republicans' difficulties in the House and conversely their opportunity in the Senate -- was underscored in a recent study by Congressional Quarterly showing that senators are generally easier to defeat than House members. Since World War II, it found, senators had a casualty rate of 32 percent compared to a 9 percent rate for House members -- partly because House members were less visible, better adapted to constitutent service and more insulated from controversial national issues.

The innate vulnerability of senatorial incumbents, coupled with the fact that two-thirds of the incumbents seeking reelection are Democrats, makes the Senate a happier hunting ground for the Republicans than the House.

The Senate has 41 Republicans and 59 Democrats (including Virginia Sen. Harry F. Byrd Jr., who calls himself an independent) -- meaning a switch of 10 seats would transfer control to the GOP.

The Democrats' most endangered species list includes Mike Gravel (Alaska), John Culver (Iowa), Frank Church (Idaho) and George McGovern (S.D.), to which the Republicans add Democrats Birch Bayh (Ind.), John Durkin (N.H.), Gary Hart (Colo.) and Warren Magnuson (Wash.).

In all, the Democrats are defending 22 incumbents and the Republicans seven, with most of the GOP senators considered safe. The Democrats look upon Republicans Robert Dole (Kans.), Barry Goldwater (Ariz.) and Jacob K. Javists (N.Y.) as the most vulnerable of the group, although all would be rated as early favorites. For the open seats -- created by the impending retirements of Adlai E. Stevenson III (D-Ill.), Richard S. Schweiker (R-Pa.), Milton Young (R-N.D.), Abraham Ribicoff (D-Conn.) and Henry Bellmon (R-Okla.) -- the Democrats believe they have a better than 50-50 chance of gaining a seat or two.

Among the many imponderables six months before the election is the impact of the presidential race on congresional contests. Many Democratic lawmakers and candidates have already put some distance between themselves and President Carter, and Republicans say they doubt the nomination of Ronald Reagan will be "another 1964," which is a nice way of referring to the Goldwater rout of that year.