REPUBLICANS HOPE in the next few weeks to pass bills in both houses that would cut taxes by about three-quarters of a trillion dollars over the next 10 years. They understand that in this Congress no such tax cut will become law. Not even House passage is certain; Senate passage could be tricky; and they lack the votes to overcome the veto that the president has indicated he would cast.

The purpose is political -- to distinguish between the parties, make the Democrats squirm insofar as they do the responsible thing and vote no, and perhaps force them to sweeten a little the smaller tax cut that they are likely to offer as proof that they, too, favor reducing taxes, just not so much. The sweetened offer then becomes the floor in negotiations later this year as the parties try to cut a deal involving tax cuts, Medicare, etc. That's the theory, and in the White House as well as among Republicans, there is guarded interest in such a deal.

But what the brokers have in mind would be a disaster -- once more a mortgaging of the economic future for the sake of a signing ceremony that would allow the politicians to look good at national expense. If the parties agree on a tax cut, it will be "paid for" out of a projected budget surplus that is almost entirely an illusion, the product of an estimating assumption that just about everyone, including the estimators, understands to be false.

The surplus is supposed to be about $1 trillion in other than Social Security funds over the next 10 years. But three-fourths of that $1 trillion would require Congress to impose a massive spending cut from just the first stages of which it already is flinching, with mostly good cause. For the surplus to materialize, domestic appropriations -- the budget for just about everything from veterans programs and Head Start to law enforcement and air traffic control -- would have to be cut more than 20 percent in real terms.

Because the president and Congress promised such a cut in the balanced budget act of 1997, the estimators have had no choice but to incorporate it into their projections. But no one thought even then that it would happen -- it was a way of making the numbers come out even without actually having to do anything -- and surely no one should think so now. When it hasn't been holding lofty discussions about how to use the surplus, this is a Congress that has been busily increasing the spending that would have to be cut for most of the surplus to occur.

A large tax cut financed by an accounting illusion, such as the Republicans propose, would plunge the government back into precisely the cycle of borrow-and-spend from which it is now escaping. It would be reckless fiscal and social policy alike. The right budget deal this year is no deal. They should pass the appropriations bills and go home; let whatever surplus develops be used, as it automatically would, to pay down debt against the day when the government will have to borrow again to cover the costs of the baby boomers' retirement. The deal-makers talk about a trade in which the president gets the Medicare drug benefit he wants in return for relenting to some degree on the tax cuts he has thus far mostly resisted. He himself has suggested a willingness to negotiate on taxes. We wish we thought he didn't mean it.