IF ELECTED president, Texas Gov. George W. Bush says he will seek a tax cut even larger than the one President Clinton rightly vetoed earlier this year as unaffordable. The governor says that the cut is well within the government's means and that he would pay for it out of a projected surplus in other than Social Security funds. But the surplus is an illusion, an accounting construct based on the altogether unlikely assumption that a future Congress will cut most domestic spending by more than 20 percent in real terms.

The candidate never mentions such cuts. It is not even clear he has been told they would be required. His own party was unwilling to support even their first approximation in the last appropriations cycle; it ended up voting instead for increases. The further increases Mr. Bush himself has proposed in spending for defense and other items make it even less likely such a surplus will occur.

A tax cut of the magnitude he proposes would thus force the government back into a policy of borrow-and-spend. That would be true even before the politicians next elected tried to deal with the looming deficit in the Medicare program. In some ways the Medicare problem makes the Social Security problem, which also has to be solved, look easy. Vice President Gore says a large tax cut would jeopardize the fiscal discipline on which the current prosperity depends. That's probably right.

Mr. Bush already has been attacked by some of his rivals for the Republican nomination for having proposed too modest a cut. Like the congressional wing of the party, they do him a favor by making him seem responsible by contrast. The real-world questions he faces come from the opposite direction. His budget doesn't add up.

He and his campaign staff went to great lengths to make it appear that his tax cut would help not just the better off, who pay most of the income tax because they have most of the income, but people at lower income levels. It would in fact help some, but much less than the accompanying material made it sound. Some of the material was close to duplicitous.

One sentence read that "half of the revenue cost of the Bush income tax cuts would finance changes designed to help low-income families enter the middle class." That turns out to be a reference to his proposals to cut the lowest tax rate from 15 percent to 10 percent while doubling the children's tax credit from $500 to $1,000 a child. It's true those provisions eat up a large chunk of the revenue, though probably less than half. But low-income people don't benefit from them at all, since they already owe no taxes, and people at lower-middle income levels benefit only a little in absolute terms. The bulk of the savings go to the better-off. More than a third of the benefit from his plan as a whole would go to the highest-income one percent of all taxpayers, and three-fourths would go to the highest-income fifth. About 4 percent of the benefit would accrue to the lowest-income 40 percent of the population.

Mr. Bush deserves credit in that he carefully stretched his plan to help some lower-middle- income families. But in the larger context of the plan, the help is marginal and was exaggerated the other day in a way that does the candidate's credibility no good. A tax cut such as this would mainly benefit a small slice of the population while jeopardizing the government's ability to fulfill much broader obligations. The making of such a proposal has become an almost obligatory part of the Republican nomination process. The Democratic candidates are dueling on health care in much the same way. They, too, propose to spend a mostly nonexistent surplus. They make the drawer sound full, but in fact it's nearly empty.