A YEAR ago, Congress and the Bush administration accomplished what for a while seemed the impossible. After months of wrangling, they came up with a credible budget agreement designed to restrain federal spending in the coming five years. They also threw in a modest package of tax hikes.A professed goal was deficit control, though some analysts quickly point out that the agreement was not geared to accomplish much in this regard. Its spending caps were set generously high. Still, the agreement has worked to keep congressional spending down at a time when the soundness of the US economy has been much in doubt. It imposed a needed discipline. It put limits on major areas of discretionary spending - defense, domestic programs other than entitlements, and foreign aid - and demanded that funding increases in benefits programs be paid for through added revenue or program trims elsewhere. Cuts in one of the major discretionary areas could not be shifted into more spending in anot her (at least not until fiscal year 1993, when that part of the agreement lapses). How things can change in a year. What seemed a credible deal last fall now appears incredibly out of date to many on Capitol Hill. Pressures to undo last year's agreement are building, particularly after the virtual disappearance of the Soviet state and its military threat to the West. Why keep a fence around a $290 billion Pentagon budget geared to a global war that has little, if any, chance of occurring? That question has undeniable logic, but last year's agreement shouldn't be hastily cast aside. Its discipline is still important. The caps should remain, though the priorities within them should be carefully rethought. The prospect of big trims in defense, a continuing economic slump, and pinched government services make the push for more domestic spending nearly irresistible. Some items that have come before Congress, like the extension of unemployment benefits, were timely responses to needs that couldn't be ignored. Others, like a House-originated bill to create a new federal student loan program for the college-bound children of middle-income families, entail large ongoing commitments. Good arguments might be made for such programs, but they should be deferred until priorities are reassessed and a new budget consensus is reached. The administration is doing its best to hold last year's agreement together. It wants to keep the high ground of fiscal responsibility. Many Democrats in Congress, on the other hand, argue that Washington's responsibility is to address long-neglected domestic needs. Both sides are going to have to give some. The budget barriers built in '90 are likely to shift next year. They shouldn't be allowed to fall. And it's to be hoped that someone remembers deficit reduction and the responsibility to ease for future generations the burden of growing interest payments on the federal debt.