IN THE FIRST round of this year's budget process, the congressional Republicans stood aside. The idea was to make the Democrats declare themselves. The Democrats had had a picnic saying what they were against: the president's budget. The fair next question, given that they were back in control of a full branch of government, was what were they for ?

The Democrats have now responsibly stepped up to that question. With hardly a Republican vote, the Democratic majorities in both houses have adopted a budget resolution that, in a levelheaded way -- half through a modest tax increase, half by restraining both domestic spending and defense -- would reduce the deficit about as much as the economy can stand. The question can now be fairly reversed. The Republicans have said what they are against. They like the president's budget no more than the Democrats do; it was put to the test in both House and Senate, and emerged with 27 and 18 votes, respectively. They don't like the Democrats' budget, either. What are they for?

This is more than an exercise in political scorekeeping. It has to do with how the country is going to be governed these last 18 months of the Reagan presidency. When former Senate majority leader Howard Baker succeeded Donald Regan as White House chief of staff last February, it was widely predicted that a new spirit of accommodation would soon suffuse the administration. On the budget, the key to almost all domestic policy, that hasn't happened.

On the contrary, both sides are now threatening scorched earth. The president says he would veto the tax increase around which the Democratic budget revolves; the Democrats, without help, lack the votes to override. They have responded that without a tax increase, there will be no inflation offset for defense, and some have also said -- as have some restless Republicans -- that as part of the forthcoming debt ceiling bill, they are prepared to put the government back under the Gramm-Rudman spending sword. The original Gramm-Rudman amendment called for automatic spending cuts in both domestic programs and defense if Congress and the president failed to reach receding deficit targets. The Supreme Court invalidated the cutting mechanism for breaching the separation of powers. Some restorers think they can safely fix that merely by stipulating the cuts in advance and requiring the Office of Management and Budget to make them.

Sooner or later some mix of legislation to keep the government going will emerge from all this bluster. The question is what kind, and how rational will it be? The longer they wait, the more they put off until some crazy deadline approaches, the worse the likely result. For several years while they were in the majority, Senate Republicans acted as brokers between the president and Democrats, helping define a constructive middle ground. They, or some combination of which they will necessarily be a part, could usefully try again. Ranking Senate Budget Committee Republican Pete Domenici did make a budget proposal earlier this year. The tax increase was smaller, but otherwise it wasn't that different from what the Democrats have come up with.

The deficit has to come down. If you ask the president how he wants to reduce it, he checks none of the above. Someone has to take the responsibility on behalf of the administration. The congressional Republicans are the only ones left