Senate Republican leaders, plagued by defections from their ranks, conceded yesterday that they probably lack the votes to win Senate approval of $40 billion in unspecified savings from Social Security over the next three years.

As Republicans vied with Democrats to be first in plucking the controversial Social Security savings item from the GOP-drafted budget proposal now before the Senate, Budget Committee Chairman Pete V. Domenici (R-N.M.) told reporters that the Social Security provision "probably cannot pass" in the form recommended by his committee.

Moreover, Republican leaders appeared divided over what to do about the proposed Social Security savings--a cornerstone of the Senate GOP's deficit-reduction plan for spending cuts and tax increases of more than $400 billion through fiscal 1985.

Senate Majority Whip Ted Stevens (R-Alaska) said the Senate was most likely simply to take Social Security--and its expected deficits of more than $20 billion through 1985--out of the budget. This wouldn't change anything, except that Social Security would no longer be counted officially in computing the budget and its deficits.

Domenici, however, ridiculed the idea, asking why Congress couldn't then exempt defense spending from the budget and claim, "My, how the deficit has improved."

And House Minority Leader Robert H. Michel (R-Ill.), who last week embraced the idea, indicated yesterday he has had second thoughts about taking Social Security out of the budget because it might look as though Congress was "manipulating numbers" to make its projected deficits look better.

As the Senate spent most of the day jockeying for partisan advantage on the Social Security issue, Michel and other Republican leaders worked behind closed doors in the House to try to find common ground for a budget agreement between GOP moderates and conservative Democrats that would be acceptable, if not totally satisfactory, to President Reagan.

Such a coalition of Republicans and Democratic "Boll Weevils" was responsible for Reagan's impressive budget victories last year. But differences over taxes and defense as well as domestic spending prevented agreement this year, at least until the House Budget Committee came up last week with a Democratic budget alternative that dissatisfied both conservative Democrats and Republican moderates.

After preliminary meetings late last week, Michel held a day-long meeting yesterday with both groups, attended as well by budget director David A. Stockman and Republican members of the House Budget Committee.

When they broke up in early evening, Rep. Buddy Roemer (D-La.) said they had "made progress," and Rep. Barber B. Conable Jr. (R-N.Y.) said they would meet again today after discussions with the groups they represent. "It's all tentative," said Conable.

Midway through the session, Michel told reporters the group was aiming for spending cuts and tax increases to produce a fiscal 1983 deficit "in the range" of the $104 billion deficit proposed by the Democratic-controlled House Budget Committee. But the mix of deficit reductions, if one could finally be agreed upon, was expected to be considerably different from the committee version--probably including less tax increases and more domestic spending cuts. Social Security, however, was not expected to be tapped for deficit reductions, according to Michel.

Michel also indicated he didn't expect a compromise from the group to be endorsed by Reagan. "I don't expect in the end to get the full endorsement of the president on what we might fashion . . . . I don't want to lock my president into something he might not embrace," Michel said.

Reagan has formally endorsed the Senate Republican version, complete with the $40 billion in Social Security deficit reductions, but White House aides, fearful of the political fallout from tampering with the popular retirement program, have indicated they won't be upset if Michel comes up with something different, which is what Michel wants to do.

Domenici's concession of likely defeat on the Social Security issue came just after he took the Senate floor to assert that Congress has a "moral duty" to tackle the Social Security solvency problem in the budget "whether it brings us trouble in the November elections or not." Said Domenici: "We are right on Social Security and the American people know it."

Domenici indicated he would try to work out a compromise that would commit Congress to action this year to keep Social Security afloat, although probably without the $40 billion in unspecified cuts, which have turned out to be a political red flag in both parties. Senate leaders plan to meet today to try to work out a compromise.