THE POLITICAL meaning of the president's annual budget is rapidly changing. You can see it in the past several days' events. In outline and in detail, the White House has once again more or less deliberately leaked the whole thing while briefing Congress. Congress is asserting a stronger part in the process, and the budget is no longer the once-a- year, once-and-for-all declaration of a president's policy and program. It has become a highly tentative negotiating position. Several congressional chairmen have solemnly blessed the president's budget as realistic. They do not mean to suggest that they endorse it. They merely accept it as a reasonable point from which to begin their own revisions--which are likely to be fully as substantial as last year's.

The budget used to be the annual demonstration of the president's domination of all the government's money matters. Lyndon Johnson took great amusement in the ceremony of unveiling it, to a great roll of drums and gasps of astonishment that all the ominous predictions, usually planted by Mr. Johnson himself, had by miraculous ingenuity been proved wrong. When the budget went to Congress, it was immediately torn apart and distributed piecemeal to innumerable subcommittees. That has now changed fundamentally. The reason for it is the budget reforms of the past decade.

The reforms have equipped Congress to carry on a sophisticated analysis of budget policy throughout the year. That has swung attention away from the fragments and back to the broad patterns that are being set. That's a good thing. But the shift of budget power to Congress is being augmented by the failure of Mr. Reagan's economic strategy, reflected in the unemployment rate.

When Mr. Reagan's budget goes to Congress at noon tomorrow, the first major revision will probably be a bipartisan jobs program. It may well look much like CETA--the Comprehensive Education and Training Act--that was enacted in the Nixon years. Like the Social Security reforms, a jobs bill will strengthen the habit of bipartisan cooperation in a Congress that may then apply it to other aspects of the budget.

Defense, in contrast, seems likely to prove divisive. It is very difficult for Congress to deal with defense spending; there are too many jobs at stake in those contracts, and too much of an impact on employers in home states and districts. The administration's recent cuts have gone after the wrong thing --the pay for people in uniform.

But the most urgent choices for Congress in this budget will be how, and how soon, to raise taxes. The deficit is now in the range of $200 billion, and even the president's own projections show no significant improvement until well after the next election. People who were frightened by last year's $110 billion are going to be twice as frightened by the figures that the White House is forecasting for this year and the next. The outlook for lower interest rates becomes less hopeful. Mr. Reagan has lost control of his budget, and now Congress will have to decide what to do about it.

The president's tax proposal--the surcharge to be considered sometime after the election, depending on this and that--will not be taken seriously. It probably is not meant to be taken seriously. The correct reading of this proposal is that many people in the administration recognize that taxes have to go up, and yet Mr. Reagan himself vehemently resists the idea. The contingent tax is a signal from the White House that it can't resolve the question at present, but at least it concedes that the possibility of a tax needs to be discussed.

That discussion may well be strongly encouraged by economic developments ahead. There is a dawning suspicion that both monetary policy and the budget may be more strongly stimulative than conventional wisdom indicates. No one is sure how the economy will respond this year. The size and timing of the next tax bill is not a question that Congress can lay aside until 1985. Mr. Reagan is refusing to deal with it. That increases the responsibility falling to Congress, as it takes up Mr. Reagan's first draft of a 1984 budget.