IN HIS SPEECH at the Republican convention, President Bush put reform of the tax code on the agenda for a second term, saying he wants "a simpler, fairer, pro-growth system." This may have been no more than an applause line. Two years ago the Bush Treasury Department examined options for tax reform and concluded that it wasn't politically salable, and Mr. Bush's ducking of Social Security reform in his first term may signal what one should expect on tax reform in a second one. But, if Mr. Bush were serious, would tax reform be a good idea or a bad one? The answer is that, in Mr. Bush's hands, it would probably be bad.

Some versions of tax reform are powerfully attractive. An across-the-board elimination of corporate tax loopholes would make the tax code simpler and fairer; moreover, by bringing current shirkers into the tax net, it would allow tax rates to be lower than otherwise, which would sharpen incentives and hence growth. If Sen. John McCain (R-Ariz.) were campaigning for president on a tax-reform ticket, we would be enthusiastic, because Mr. McCain has a record of resisting lobbyists' pork-barreling. But it's hard to believe that Mr. Bush means to pursue this kind of tax reform. He has shown little willingness to stand up to corporate lobbyists so far, refusing, for example, to threaten a veto of the loophole-filled corporate tax bill that Congress is finalizing.

What else might tax reform mean? It could entail elimination of loopholes created not by lobbyists but by social policy: tax breaks for charitable donations, child credits and so on. There is a case for such reform: Tax credits create complexity, force tax rates higher and lower the growth rate, so you have to feel strongly about the social goals these credits are meant to achieve. But again, it seems unlikely that Mr. Bush intends this version of tax reform. He is proud that the child credit has doubled on his watch. He is not about to tamper with the popular right to deduct all mortgage payments, even though there is no justification for subsidizing families with mortgages of $300,000 and more.

Mr. Bush's talk of tax reform probably signals something different: a continuation of his effort to cut taxes on saving and concentrate them on consumption. The president already has cut the dividend and capital gains tax rates, and he has promoted various tax-exempt savings accounts; a second term might bring consolidation of this work. The aim is to encourage Americans to save more and therefore to make capital more plentiful. That in turn might boost corporate investment and hence economic output.

There are problems, however. Eliminating taxes on savings makes the tax code less progressive, since rich people do the lion's share of the nation's saving. It deprives the government of revenue, perhaps draining money from education or other programs that could enhance growth more than additional savings could. There is considerable doubt, moreover, about how much savings would increase in response to tax cuts: Some households might figure that they could achieve goals such as paying for their children's college tuition by putting less aside, and so savings might go down. For these reasons, savings incentives should take the form of tax-exempt accounts available only to low-income people.

Perhaps we have guessed wrong about what Mr. Bush really means by tax reform, and perhaps we would support his plan if we knew more about it. The president still has a month and a half to flesh out his thinking. Meanwhile, he could demonstrate a seriousness about resisting awful tax loopholes by vetoing the corporate tax bill that is on its way to his desk.