Democrats on the House Budget Committee last night agreed on a fiscal 1986 budget plan that would trim $56 billion from federal spending without cutting Social Security or raising taxes.

A one-year freeze on Social Security was a key part of the $56 billion deficit-reduction plan passed 50 to 49 by the Republican-controlled Senate early Friday.

The House Democrats' plan would also freeze defense spending with no increase for inflation, and reduce many domestic spending programs while protecting those targeted at the poor. The Senate was more generous with defense, providing funds to cover inflation.

House Budget Committee Chairman William H. Gray III (D-Pa.) declined to provide specifics of the plan last night, saying he would release details today, when panel members from both parties meet to begin marking up the fiscal 1986 budget resolution.

However, Gray indicated he thought the Democratic plan, worked out in closed sessions over the last few weeks, was fairer than the one passed by the Senate with White House approval.

Because Democrats control the Budget Committee 20 to 13, the Democratic plan is likely to be sent to the House floor, but Gray said he did not know if it would be approved there.

According to Budget Committee Democrats, who spoke before the final vote on the plan last night on condition that they not be identified, they used the additional savings in defense to protect domestic programs President Reagan had proposed to eliminate, including mass transit subsidies, Amtrak, Urban Development Action Grants (UDAGs) and the Small Business Administration.

However, those programs still would be reduced substantially by the Democrats. For instance, UDAGs would be cut by about 10 percent under the plan, and Amtrak by about 7 percent, or about $50 million, Democrats said.

They also said that revenue sharing with local governments would be reduced by 25 percent in fiscal 1986 and then eliminated. Mass transit subsidies would be frozen, lawmakers said.

At the same time, education funding would get a small increase, and means-tested programs targeted at the poor, such as Medicaid and food programs, would receive enough funds to cover inflation, Democrats said.

A tentative agreement to give federal workers a pay increase was killed.

Gray said last night that, in addition to protecting senior citizens by refusing to freeze cost-of-living adjustments (COLAs) for Social Security and other retirement plans, Democrats on the Budget Committee had rejected the "tremendous" cuts in Medicare approved by the Senate.

The Senate voted to reduce funding for Medicare, the health-care program for the elderly, by $16.3 billion over three years by freezing payments to doctors and hospitals, and increasing premiums.

A Washington Post-ABC News poll showed yesterday that the public opposes reductions in COLA increases for Social Security recipients by a 4-to-1 ratio. Support for defense cuts, however, is growing, with 57 percent of those polled saying "substantial cuts" could be made there.

Democrats, meeting in a closed caucus, came out strongly against a freeze on Social Security payments.

"The Democrats are opposed to cutting benefits 4 to 1," House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) said. The caucus did not take a vote on the issue because the sentiment was so overwhelming and a vote would only "embarrass" some Democrats, he said.

Other lawmakers said a handful of conservative and moderate Democrats argued for a one-year freeze on all retirement-benefit programs as the only fair way to make deep deficit reductions.

But dozens of others took the podium to say Social Security has been the one issue that has helped the Democrats win elections over the last four years and that giving it up would be political suicide. Democrats picked up 26 House seats in 1982, in part because of concerns that the Reagan administration would cut Social Security.

Rep. James L. Oberstar (D-Minn.), one of those demanding the caucus, told the lawmakers that Social Security was "our principal Democratic political capital" and that House Republicans were praying that the Democrats would abandon Social Security so that the GOP could seek to save it.

Later, some conservatives sharply criticized their colleagues as playing politics instead of trying to deal with the deficit.

"The numbers demand that with compassion you touch Social Security . . . but the politics deny it," Rep. Buddy Roemer (D-La.) said. "There are in the average member's district 70,000 Social Security checks. That's a pretty large special-interest group."

Several conservatives also indicated they would have a difficult time supporting a Democratic budget plan that left out Social Security while making deep cuts in other programs, such as defense. In 1981, southern Democratic conservative "Boll Weevils" bolted from the party to vote with the Republicans to pass Reagan's plan