House Democratic leaders, savoring victory after forcing President Reagan to back off his proposed long-term cuts in Social Security benefits, said yesterday that they weren't sure what to do about the bipartisan study commission he now wants to create.

They fear that it would be stacked in his favor, would also recommend cuts, but would provide him with political cover on the sensitive issue.

House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) and Majority Leader James C. Wright Jr. (D-Tex.) were careful not to turn down the president's proposal, which was to create a 15-member commission with five appointed by O'Neill, five by Senate Majority Leader Howard H. Baker Jr. (R-Tenn.) and five by the president. But they indicated that they may seek some changes in its makeup.

"I don't see how we can refuse the president a commission like that," said O'Neill, but quickly added that he was bothered by the possible 2-to-1 Republican majority. "I want a fair commission," he said, and not one that would "want to balance the budget no matter who it hurts."

He said he would want to know more about it before going along. "If we are to form a new commission it is vital that this new panel be committed to the future of Social Security."

Wright was even more blunt. "I don't think Tip O'Neill or I would be in favor of a proposal that would enable Republicans to name 10 of the 15 members," he said.

Baker, in an interview, indicated that he viewed the commission idea as a constructive suggestion. To relieve O'Neill's fears, he would urge that the House and Senate leaders each name three majority party and two minority party members and that the president split his choices three to two, so that at most there would be eight Republicans to seven Democrats.

With the president's Thursday night speech and a letter to O'Neill, the whole Social Security issue took on a new character. A long-range bill making big cuts seems dead for this year, and Democrats are claiming the credit and believe they have scored a major political victory.

The president, in a letter to O'Neill dated Sept. 24, specifically withdrew the assortment of long- and short-range cuts he proposed May 12, although he repeated that he still believes his plan would be a "fair, balanced and workable solution" to the financing problems of the system, which has 36 million beneficiaries and costs about $145 billion a year.

His plan had been heavily attacked by Democrats on grounds that it cut far more than needed to shore up Social Security, and was in part an effort to use Social Security to balance the budget. Democrats contend that interfund borrowing among the three Social Security trust funds will be enough to meet immediate cash flow problems.

O'Neill had said he would not allow any bill with large cuts to come to the floor this year. The president, in his letter, conceded that no long-range comprehensive bill could be passed "in the immediate future" and told O'Neill, "I am therefore proposing that the Congress postpone action on my plan" and join in a study commission that would bring back bipartisan suggestions "in the next one to two years."

Meanwhile, Reagan said he would agree to a stopgap solution of allowing the cash-short old age trust fund to borrow from the better-off disability and Medicare funds for the next few years.

He also said that Congress should restore the minimum benefit, which it repealed at his request last month, for low-income people who are already on the minimum rolls.

The Senate Finance Committee, a few hours before the president's speech, had approved a bill Thursday along those lines by a 19-to-0 vote, and the measure is expected to pass the Senate next week.