Preliminary struggles over the fiscal 1986 budget resolution in the House demonstrate that the once-dominant liberal wing of the Democratic majority has been reduced to a peripheral force, strong only among the Congressional Black Caucus.

"When I came here 10 years ago, I voted for job stimulus and for what was then a pretty big deficit because the deficit was supposed to stimulate jobs," said Rep. James L. Oberstar (D-Minn.), chairman of the Democratic Study Group (DSG), an organization reflecting the ideological flux of the Democratic left.

"Now we find our gains of 10 years ago eroded," he said.

The DSG has made deficit reduction its central priority. "If we don't have some means of whittling down the deficit, all we will be talking about for the next 10 years will be further spending cuts," Oberstar said.

The budget choices facing the Democratic-controlled House illustrate this new equation.

Of the four major budget substitute amendments, two are Republican proposals with sharp domestic spending cuts and a third is a Democratic initiative that would freeze Social Security while maintaining the $56 billion in deficit reduction approved by the House Budget Committee. Not even the Republicans are willing to tamper with Social Security.

The fourth is the black caucus alternative, the only proposal that could be described as "liberal." The caucus alternative won Rules Committee approval for floor consideration more as a courtesy to its authors than as a proposal that has a chance of passing.

"I don't want to use the word 'isolated' to describe how I feel," said Rep. Parren J. Mitchell (D-Md.), a former black caucus chairman. "The 'new' liberals, the Leon Panettas and the Richard Gephardts both Democratic leaders of anti-deficit drives consult with me regularly, but I just don't feel anywhere near as comfortable with the ideological direction the party is taking nowadays."

When he was first elected in 1970, Mitchell said, there were 160 to 180 members who could be counted as firm liberals; now, he said, there are little more than 100.

At the start of this week, three groups of Democrats were calling for inclusion of a corporate minimum tax in the budget resolution. None of the proposals, however, including the one advanced by the DSG, would have used the revenues to restore domestic spending cuts in the House Budget Committee's resolution.

In fact, in putting its emphasis on reducing the deficit, the DSG has subordinated tax reform, historically the goal of liberal Democrats.

"Robert Taft is probably turning over in his grave, laughing," a Democratic Ways and Means Committee aide said in reference to the late Ohio conservative senator and noting the ideological shift of Democrats.

The House Budget Committee resolution seeks to fully protect programs directed specifically at the poor, but it calls for cuts in rural housing, the Small Business Administration, community development block grants, Urban Development Action Grants, agriculture programs and elimination by 1987 of revenue sharing.

"There are a lot of things in the House budget I don't like," Rep. Howard L. Berman (D-Calif.), a liberal serving his second term, said. "But I don't think the things I would like have 217 votes."

Calling for use of a minimum tax to finance lower individual rates in tax revision legislation, Ways and Means Committee Chairman Dan Rostenkowski (D-Ill.) criticized those who would use the money for deficit reduction and rhetorically asked: "Do we want to lower rates so a Democrat, who has been part of our constituency, gets more money in his pocket?"

His comment suggested that Democrats may shift from spending programs to tax policy as a vehicle to provide support to lower- and middle-income groups.