Since President Bush has ''opened the door to a {budget} deal,'' The Post seems ready to look no farther for any real leadership from him to find a solution to the federal budget problem {"A Useful Budget Blur," editorial, May 21}. More leadership can and should be expected.

Since the beginning, the governing concept has been that the president proposes; Congress disposes. That basic idea is explicit in the laws that prescribe the federal budget system. I spent 30 years in federal budgeting at the Bureau of the Budget (now the OMB) and as budget director for the Transportation Department, and I know that if economic or other developments require changes in the president's budget submitted in January, which is the case now, the president is supposed to submit them to Congress in the form of budget amendments. This would be nothing new or novel; it has happened countless times in the past. The only difference this time is that the changes might require new taxes, a fact this particular president refuses to acknowledge.

The budget crisis cries out for presidential leadership. That leadership is not fostered by blurring responsibility and accepting the president's assent to deal as ''progress." The Democrats would be wrong institutionally and foolish politically to let him get away with it. ROBERT G. PRESTEMON Rockville