THE PRESIDENT and congressional Republicans have just piously agreed to try not to spend billions of dollars they're already busily spending. They say the Social Security surplus should not be used to finance the rest of government. But stripped of gimmicks, the current round of spending bills will likely consume more than a fifth of this year's surplus -- upward of $30 billion.

You could argue that the posturing is harmless. Social Security is neither worse nor better off. The same IOUs are put in the trust fund whether the surplus is used to finance other programs or to pay down debt. If pretending that they're not using the Social Security surplus helps them pass the appropriations bills and go home, what could possibly be wrong with that?

But it's not that simple. The pretense -- misrepresentation, distortion, fib, lie, call it what you will -- perpetuates a set of myths about the budget and Social Security whose effect is to make it harder to deal rationally with the government's long-term fiscal problems. The gimmicks being used to support the lie will meanwhile cause enormous budget problems in the years immediately ahead. To make the budget come out even next year, an election year, even more gimmicks will be required -- bigger lies.

A couple of years ago, to grease another deal -- the Balanced Budget Act of 1997 -- the president and Republicans came up with a similar cover. The budget already was on the way to balance, thanks to a strong economy. They wanted to add a tax cut. To create the appearance of having financed it, they agreed to artificially tight appropriations caps in the years ahead. No matter that the caps implied budget cuts neither party was prepared to vote; they'd worry about that later.

It is largely those caps that produced the illusion of a budget surplus, in other than Social Security funds, over which the parties have been fighting all year. The Republicans wanted to use it to finance their tax cut, the president to cover his own initiatives. But it doesn't exist. They've spent the entire year arguing about the use of an accounting construct, or to put it another way, the president and the Democrats have spent the entire year wriggling free of the consequences of a previous lie.

Paying down debt is a laudable use of the Social Security surplus, or as much of it as will remain after paying the government's other bills. It shows up as an increase in national savings that should stimulate economic growth. Paying down debt now will make it easier to borrow again when the baby boomers retire. But it's not a raid on Social Security to use the surplus for other purposes, as both parties have voted to do for years. And the gimmicks being used to make this year's spending look less than it is -- above all the shifting of billions of dollars into next year -- will only make that year's problems worse.

The Republicans sense two advantages in insisting that spending for other than Social Security not exceed non-Social Security taxes. They can claim to be protecting Social Security and put a tight lid on the rest of the budget, all in the same stroke. No matter that they aren't protecting Social Security in the sense they suggest, or that they lack the votes even within their own caucus for the spending cuts the new principle implies.

The president has chosen for political reasons not to argue. He says he, too, wants to govern without using the surplus. As with the appropriations caps in the 1997 act, he thereby joins for short-term reasons in setting a phony standard that he no more than they will likely be able to meet next year, any more than this one. But next year will be time enough to wriggle out of that one.