Just three months ago, the Democrats exploited President Reagan's vulnerability on Social Security and unemployment during the 1982 congressional elections to solidify their control of the House and give the Republicans a scare in the Senate.

But since then, they have joined Reagan in a compromise on Social Security financing and have neared an accord with him on legislation providing jobs and humanitarian aid to victims of the recession.

They have nudged Reagan into a rather dramatic departure from his basic economic tenets of lower taxes and reduced spending, forcing him to accept both tax increases for Social Security and accelerated spending for jobs. While still insisting he was against "make-work" jobs, the president embraced important elements of an employment package that he had forced Congress to drop under threat of a veto just two months ago.

But the Democrats appear to have paid a high political price for this. By opting for compromise instead of confrontation, they have let Reagan slip out of a difficult corner and blunt their argument that he is insensitive to the needs of the elderly and the unemployed.

On jobs in particular, Reagan succeeded in sidestepping a bipartisan steamroller and offered just enough to rekindle cooperative instincts in the Democratic House and Republican Senate, where fears of political gridlock run deep.

Arguments persist over whether Reagan has moved sufficiently, let alone willingly. But he gets more attention than his critics do on television as he earnestly discusses the administration's role in achieving the compromises on Social Security and a jobs package.

Many Democrats, including the chairmen of their House and Senate campaign committees, agree that Reagan has benefited politically, at least in the short run.

"Every president has the opportunity to box in the opposition, and this president is doing it very, very effectively," said Rep. Tony Coelho (D-Calif.), chairman of the Democratic Congressional Campaign Committee.

With deepening troubles in the GOP as well as in foreign and domestic policy, the president "could not suffer any more losses and still govern," Coelho said, so he shifted course just as he did when he was governor of California.

"I'm not being critical," added Coelho. "I really do admire him. I admire his ability to turn and go."

Sen. Lloyd Bentsen (D-Tex.), chairman of the Democratic Senatorial Campaign Committee, agreed that Reagan has profited from his post-election dealing with the Democrats.

"A large part of his problem is an image of lack of compassion, and this change of course obviously does help blunt the issue," said Bentsen.

Even if it hurts politically, Bentsen and Coelho agreed that the Democrats have little choice but to cooperate, especially when the president moves--in substance if not in rhetoric--in their direction.

"Of course, it's risky but the objective of Social Security solvency and jobs creation is too important," said Bentsen.

"Democrats are in a box," said Coelho. "If we say no, if we say it's too little, then we look like the problem . . . . There's not much we as Democrats can do about it when it what the president is doing is right."

A key problem for the Democrats, especially after regaining effective control of the House, is that they feel a pressing need to accomplish something. But they still face a Republican Senate and a Republican president, whose veto has been overridden only once on a major bill.

House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) put the dilemma this way in discussing Reagan's $4.3 billion jobs and recession-relief offer:

"Clearly we Democrats want to do more than what the president proposes and in fact we will make additions to his bill. But we are not looking to make political points by provoking a veto. We are looking to enact a jobs bill into law. There are too many people truly suffering to pass up an opportunity to alleviate the suffering of some of them right now."

As Coehlo sees it, the White House is trying to create an impression of momentum to keep the Democrats off balance. "What they're trying to do is get a roll going," he said. "If they get some momentum going, they think they can offset us."

Others on Capitol Hill think the White House objective is basically damage control in preparation for new attempts to squeeze social welfare spending, build up the military and hold the line on taxes.

Under this theory, Social Security financing reform is such a political liability that the opposition can be neutralized easier by inclusion than exclusion. And, with jobs legislation appearing increasingly inevitable in light of persisent double-digit unemployment, a modest early initiative might deflect more costly ventures and clear the decks of a difficult problem before the bigger budget fights begin.

But even if the Social Security and jobs compromises are enacted as expected, it is by no means certain that Reagan has his economic program back on track.

One high-level Republican committee staff aide said Reagan's concessions on jobs actually may undercut his efforts to contain social welfare spending. "These guys up here have a very keen sense of political strengths and weaknesses," he said. "How can you have a 180-degree reversal on one thing and and not expect it to affect anything else? "

As if to prove the point, conservative Rep. John N. Erlenborn (R-Ill.), head of a GOP task force on jobs, told reporters that a "phase 2" Republican jobs program may include mortgage and health insurance protections for the jobless, expansion of food stamps, more government spending on job retraining and other initiatives that also are backed by Democrats.

Other Republicans, such as Sen. Dan Quayle (R-Ind.), chairman of the Labor and Human Resources employment subcommittee, scoffed politely at administration contentions that accelerated jobs spending in the $4.3 billion proposal would have to be offset in future budgets. Democrats said the administration was simply pipedreaming.

But some Democrats were nonetheless worried that Reagan's initial jobs offer might succeed in taking some steam out of the "phase 2" drive. That, said Sen. Carl Levin (D-Mich.), head of the Senate Democrats' jobs task force, is why Democrats are pushing for more in the "phase 1" bill--which Levin contends is replete with spending proposals that don't really add money for jobs.

"Phase 2 is a wish," said Levin. "The administration will be able to say, 'Hey, we did something about jobs, and now here come the Democrats with the big-spending boondoggles.' . . . Okay, if they want to be perceived as wanting to do something about jobs, then we ought to be able to extract something in return: jobs."