For nearly three years, the once mighty Senate Democrats have been the lost tribe on Capitol Hill, forced into the wilderness by Republicans in their own chamber, by the big Democratic majority in the House and at times even by the small House Republican minority.

Now they are wandering no more.

Even before the November elections next year that could return them to power, the 45-member Senate Democratic minority has moved with unaccustomed force and unity to seize the initiative on a critical issue and become the driving force behind a major congressional confrontation with the White House.

The issue is whether President Reagan must share power with Congress, under the Vietnam-era War Powers Resolution of 1973, in determining whether, how long and under what conditions the U.S. Marines may remain in Lebanon.

House Democrats have been pulled one way, then another, on the issue as their leadership ran into a back-benchers' revolt when it tried to take a conciliatory position.

Senate Republicans, especially Majority Leader Howard H. Baker Jr. (R-Tenn.), have been caught in the increasingly tight squeeze of trying to reconcile a clash of institutional claims between Congress and the White House.

House Republicans, who almost ran the House in league with Democratic conservatives in the first two years of the Reagan administration, appear to have gone underground.

Into this power vacuum the Senate Democrats have stepped with a unity, command and sense of mission that has generally escaped them since Reagan swept into office, bringing with him a Republican Senate majority for the first time in a quarter-century.

Senate Democrats participated as full partners, or nearly so, in the budget battle earlier this year that produced a compromise that Reagan wouldn't accept, a kind of congressional declaration of independence from the White House on budget priorities. But the point men of that struggle were Republican moderates; the budget was their trophy.

On Lebanon, however, "without any question, the Democrats have taken the lead," conceded a high-level Republican aide.

What they have done is to demand that Reagan acknowledge that U.S. troops have been in "hostilities" in Lebanon since two Marines were killed there Aug. 29. Such presidential notification would trigger a requirement under the War Powers Resolution that Congress authorize continued deployment of the Marines within 60 to 90 days.

Reagan thus far has refused to do this, although Republicans in increasing numbers say he eventually must do so and one of them, Sen. Charles McC. Mathias Jr. (R-Md.) has introduced a triggering resolution of his own.

While there are political risks on both sides of the confrontation, including a possible backlash against the Democrats for "meddling" in foreign policy and encouraging intransigence on the part of the Syrians and other hostile forces, there is a fear running through GOP ranks that Reagan may have finally overreached his grasp on Congress.

"There's a fear he's losing whatever ground he gained from his handling of the Korean plane disaster," said a Senate Republican leadership aide.

Also, some lawmakers in both parties have expressed concern that continued squabbling over war powers could undermine congressional support for the Marine deployment, especially if hostilities worsen.

Separating the war powers struggle from other issues is that fundamental executive and legislative powers are at stake, which tends to unite lawmakers regardless of party and philosophy.

Moreover, the War Powers Resolution itself was the product of a bitter struggle in that arena, achieved a decade ago in the waning days of the Vietnam war over the veto of President Nixon.

The Republican leadership is inhibited from an independent role in the matter because of its obligations to a president of its own party, although there is clearly pressure on Baker to act more forcefully before things get out of hand.

The Democrats have no such political burden. Moreover, they claim to feel confident of the support of enough Republicans to carry the day in the Senate as well as the House, if a vote is forced.

Suspicious Republicans, pointing to the fairly prominent involvement of Senate Democratic Campaign Chairman Lloyd Bentsen (D-Tex.) in the dispute, suggest that the Democrats are doing push-ups for 1984 to show that they can, indeed, run the whole show in the Senate.

To this, Sen. Thomas F. Eagleton (D-Mo.), an author of the War Powers Resolution and key figure in the current fight, responds, in effect, nonsense. "We don't sit there in that caucus, sort of drooling and saying, 'Ah, our day in the sun is inevitably coming and so we'll now have our spring training exercises for taking control,' " Eagleton said last week.

What motivates the Democrats, Eagleton said, is a "deep, deep commitment" to the War Powers Resolution stemming from the Democrats' decisive role in passing the legislation.

Although Republicans have been predicting that the Democrats will become more assertive as the 1984 elections approach, especially if they sense a chance to regain control, the impact of the impending elections on the war powers debate is indirect at best.

Like a candidate who seems to hit his stride when the polls show him a winner, the Democrats seem to speak with greater authority and confidence on the war powers issue than they have on other matters.

Among other political signposts, Minority Leader Robert C. Byrd (D-W.Va.) is positioning himself ever more prominently, including renewing the periodic sessions with reporters who cover Capitol Hill, held when he was majority leader.

But, with the late Sen. Henry M. Jackson (D-Wash.) replaced by Republican Daniel J. Evans (R), who appears favored to retain the seat in this November's special election, the Republicans now hold a fairly comfortable 55-to-45 advantage.

To be sure, the GOP probably will be defending 20 seats next year, as opposed to 13 for the Democrats; two Republican incumbents from basically Democratic states, Baker and Armed Services Committee Chairman John G. Tower of Texas, will not be seeking reelection.

A third popular Republican incumbent, Appropriations Chairman Mark O. Hatfield of Oregon, is known to be weighing the unappetizing thought of serving again as a minority party senator as he decides whether to seek reelection. Foreign Relations Chairman Charles H. Percy (R-Ill.) has had similar concerns, although aides say he definitely is running again.

The only Democratic senator known to be planning to retire is Jennings Randolph of West Virginia, which is strongly Democratic.

Despite all of this, Republican aides said earlier that pessimism about 1984 is beginning to fade as the economy brightens and as Soviet behavior rekindles hawkish views about national security issues. This "could even save Jesse Helms in North Carolina," another GOP aide said.

No matter whether the Democrats are positioning themselves for the next elections and thereafter, at least some of them are clearly looking beyond Lebanon.

Said Eagleton: "Another reason for this deep feeling in the Democratic caucus on war powers transcends Lebanon. There's a strong feeling in the caucus that someday we may be faced with American troops in El Salvador. Rightly or wrongly we're worried about that possibility. Therefore, if the War Powers Act is frustrated in this instance, it may well be frustrated in some future instance . . . . If we lose the War Powers Act in this instance, we may lose it forever."