The chairman of the Senate Budget Committee warned yesterday it will be all but impossible for Congress to go along with President Carter's plan to trim the budget deficit below $30 billion if the economy goes into a recession next year.

Sen. Edmund S. Muskie (D-Maine), the chairman, said Carterhs commitment to pare the deficit to less than $30 billion "has been oversold" and "may have been made too firm." He said if the economy slumps in 1979, "the $30 billion figure isn't going to be possible."

Muskie's remarks, at a breakfast meeting with reporters, amounted to a sebtack for the administration's anti-inflation plans, and appeared to heighten the debate among mainstream Democrats over whether Carter's new austerity policies were necessary.

Carter appeared to prevail after a midterm party convention in Memphis last weekend backed his budget-cutting plan. The president later told a news conference that "a 60 percent margin" of delegates had supported him, and "I am going to cut the budget deficit."

Yesterday, Treasury Secretary W. Michael Blumenthal reaffirmed that pledge forcefully before the congressional Joint Economic Committee telling the panel: "Let me repaet that the president intends his 1980 budget to be tight, with a deficit of $30 billion or less."

Although the White House is still insisting there won't be a recession in 1979, many private economists are predicting a measurable one, with the unemployment rate likely to rise from the present 5.8 percent level to 6.5 percent or higher.

Muskie said yesterday that while most members of Congress essentially support the President's anti-inflation moves, they would be unlikely to go along with a tight budget if it appeared the economy were in trouble. The Budget Committee drafts Congress' won budget tragets.

He also expressed serious doubts that the lawmakers would go along with Carger's rewuest to forgo any further cuts in income taxes until the budget finally is balanced. The president urged such a move as part of his Oct. 24 anti-inflation speech.

The administration ran into an additional congressional challenge yesterday on its pledge to trim the deficit to below $30 billion-this time over wheter it even would be possible to reach that target undeer current White House economic assumptions.

Rep. Henry S. Reuss (D-Wis.), chairman of the Joint Economic subcommittee on international affairs, said the deficit figure is based on predictions that inflation will slow to 6.5 percent next year-a forecast he charged was unrealistic and would require an overall economic growth rate of 5.5 percent.

Charles L. Schultze, chairman of Carter's Council of Economic Advisers, said he didn't have the figures handy to dispute Reuss' contentions, but argued that "we don't see the need for anything like that" rate of growth to achieve the $30 billion target.

The aministration currently is forecasting that the economy could slow to a 2 to 3 percent growth rate in 1979, with inflation ebbing to 6.5 percnet-far different from outside economists' predictions of a 1.5-to2.5 percent growth rate and an inflation rate of 8 to 9 percent.

Analysts have warned that the forecast itself could bring on a spate of problems. If the private economists are correct, Carter's budget could end up several billion dollars off the mark. At the same time, if Carter were to agree with their forecasts, he would wind up predicting a recession and the failure of his wage-price program.

In the latest of the private forecasts, the University of Pennsylvania's Wharton Econometric Associates predicted a 1979 growth rate of 1 to 2 percent, with an inflation rate of 7 percent or higher. But it warned that several potential pitfalls "could make the outlook worse."

Muskie said the Congressional Budget Office has predicted tentatively the economy will grow by about 2.5 percent nex year. He said the agency has concluded that trimming the deficit to $30 billion "would be tough cutting, especially with a 3 percent rise in defense spending."

On other issues, Muskie predicted that Carter would find "very receptive ears" in Congress if he proposed revamping the revenue-sharing program to shift more money to cities, rather than states. "If he moves in that direction," he said, "I think he will . . . get some support."

Muskie also dismissed suggestions that Congress might delay or roll back shceduled increases in the minimum wage or in Social Security payroll taxes, even if Carte were to propose them. The president disclosed Wednesday his advisers are considering a special reduced wage for youths.

Muskie made his comments at a breakfast meeting with reportes. Rep. Robert N. Giaimo (D-Conn.), chairman of the House Budget Committee, was not available for comment. CAPTION: Picture, SEN. EDMUND S. MUSKIE . . . voices doubts on deficit figure