Tomorrow President Reagan will address Congress in behalf of his economic program. A day or two later, the budget resolution is to go to the floor of the House. It will not be in the form the administration wants, having been changed in several important respects by the Democratic leadership. The Republicans will try to recruit enough dissident Democrats to carry the president's original version. The crucial floor votes are now scheduled, very tentatively, for Friday.As the first dramatic tests of the new administration's strength in Congress, these votes promise to cast a long shadow. What, precisely, is at stake?

As usual, there's more than one answer. As a matter of purely adversary politics, the administration knows that one victory tends to lead to another. A rising impression of strength and control would be helpful to the adminstration in coming challenges that have nothing to do with budgets or money.

But the immediate issue is the budget, and here the stake is clearly the tax reductions and the size of the federal deficit. On the spending side of the budget, the differences between the administration and the Democrats are hardly visible without a magnifying glass. To hold their more conservative colleagues, the Democratic leaders have retreated from their earlier idea of a less large increase for defense. The Democrats' budget does more to protect some of the social programs, but even there the differences in dollars are not great.

Tax policy is another matter. Since the administration wants a larger reduction in taxes, it has to defend a much largerr deficit than the Democrats do. That posture makes a good many congressional Republicans desperately uncomfortable, and in tactical terms the deficit is the vulnerable point of the president's program. The administration's estimate for its deficit in fiscal 1982, the year beginning next October, is $45 billion, and the Democrats say, with very good reason, that it would be at least $50 billion. The Democrats' counter-budget would run a deficit of only $26 billion. Mr. Reagan's program also has large implications for the following years, since it includes further tax cuts in 1982 and 1983. This schedule of successive annual tax cuts raises an obvious threat of uncontrollable deficits later in Mr. Reagan's term.

For any congressman but a blind partisan, the decision on the budget resolution is going to be a close and difficult one. People in both parties have recognized the cost of weakened presidencies in the past decade and see a need for a stronger and more assured executive. It's early in Mr. Reagan's term to rebuff him on an issue of central interest to him. But the three-year tax strategy is defective and, if enacted, it will get Mr. Reagan -- and the country -- int the most serious kind of trouble. On taxes the Democrats are right.If the budget resolution is passed in the administration's form, it will not -- fortunately -- preclude Congress from going back immediately to improve the tax provisions.