As presidential aides unveiled the 1983 budget, top economic adviser Murray L. Weidenbaum cautioned that the figures ''are based on the assumption that Congress will approve the expenditure and revenue proposals.''
That is assuming a lot, based on reactions so far on Capitol Hill. The budget projects the biggest deficits in history, even if Congress is willing to make another series of painful cuts in social programs, while defense spending continues to expand. And the Reagan proposals for closing tax loopholes are finding a mixed reception among congressmen.
Democrats, after years of dodging Republican potshots over high deficits, now have the ammunition.
''The biggest disappointment is that very large deficits appear to continue, '' said House Budget Committee chairman James R. Jones (D) of Oklahoma just after the President's spending package was released over the weekend. He charged that the red ink would keep interest rates up.
''This time last year the Reagan budget projected a $42 billion deficit'' for 1982, said Sen. Bill Bradley (D) of New Jersey, a member of the Senate Finance Committee. The new budget puts the deficit at twice that much, $98.6 billion. Senator Bradley questioned the prediction that next year's deficit will be only
Bradley and other Democrats have called for reconsidering the third year of the tax break for individuals and the newly enacted tax breaks for corporations.
However, Congressman Jones declined to promise that his party would come up with its own solution to the nation's fiscal woes. ''The only meaningful way is bipartisan,'' he said. ''We are not going to get one party to stick its neck out unless the other party joins in.''
House minority leader Robert H. Michel (R) of Illinois acknowledged that ''the President's budget will be difficult to pass in its entirety,'' but he predicted that the final version will reflect the President's priorities.
Mr. Michel, widely praised as a key to the Reagan victories in the last session, voiced serious concerns about the ballooning deficit, which he blamed on years of ''neglect'' under Democratic government.
The President is ''on the right course,'' said Michel. ''The public is very much committed to that course.''
During the next four to five months, at least, Congress will spend much of its time haggling over the tough issues in the budget. Rep. Barber B. Conable Jr. (R) of New York, ranking Republican member of the House Ways and Means committee, offered a grim prediction to reporters last week over breakfast. ''The problem is that none of the choices are attractive or popular,'' he said of the deficit. The only options are raising taxes more or cutting programs more than President Reagan proposes, he said.
Among the major issues lawmakers will face:
* Big deficits. Republicans are putting the blame on a sudden downturn in the economy, which boosts government costs and cuts revenues, as well as on past years of excess. Democrats blame the tax breaks and defense spending.
* Defense spending. Even some of the most hawkish members of Congress are beginning to ask for savings here. But some Republicans and Democrats say that the President will probably win most of his increases anyway.
* Social program cuts. In an election year, these will be even more controversial than last session. As several Democrats and northern Republicans point out, constituents are already feeling the sting from the last round of cutbacks.