IN A LETTER on this page today, Glenn Hubbard, dean of the Columbia Business School and an adviser to Republican presidential candidate Mitt Romney, takes issue with an editorial we published Sunday, headlined “Mr. Romney’s ‘garbage.’ ”

Mr. Romney claims he can reduce income tax rates while not reducing revenue or hurting middle-class taxpayers. This is a central argument of his campaign, so it’s worth reexamining in light of Mr. Hubbard’s response.

Mr. Romney says he will abolish the estate tax, permanently extend the Bush tax cuts, get rid of capital gains taxes for families earning $200,000 or less, reduce the corporate tax, eliminate the alternative minimum tax and reduce income tax rates by 20 percent so that, for example, the top bracket will go from 35 percent to 28 percent. He says he can accomplish this reduction in rates without lowering revenue by “broadening the base,” or closing loopholes. He hasn’t said which loopholes, maybe because the most expensive ones are also very popular: Among them are tax breaks for mortgage interest, charitable donations, state and local income tax and employer-provided health care.

The Tax Policy Center (TPC), a joint venture of the Urban Institute and the Brookings Institution, examined Mr. Romney’s claim and found that, even if every loophole for the top brackets were closed, there wouldn’t be enough revenue. The middle class would have to pay more.

Mr. Romney called that analysis “garbage.” Mr. Hubbard says it’s “simply incorrect.” What’s his case?

First, he says the TPC ignored the effects of corporate tax reform. That’s not quite true; the report assumed Mr. Romney’s corporate tax reduction would be revenue-neutral, which worked to Mr. Romney’s advantage. Given his proposed reduction in corporate tax rates, had the TPC taken this into account, “the result would have been an even larger tax cut on high-income individuals,” the TPC argued in a follow-up report.

Mr. Hubbard says the TPC analysis didn’t account sufficiently for changes in individual behavior and economic growth. In fact, since broadening the base would offset the rate reductions, the authors of the TPC report — Samuel Brown, William Gale and Adam Looney — found there wouldn’t be much effect on growth. But even accepting a different analysis, from pro-Romney economist Greg Mankiw, they found it wasn’t enough to let the middle class off the hook.

Because Mr. Romney said he doesn’t want to reduce incentives for savings and investment, the TPC assumed that tax breaks for life insurance and municipal bonds would be safe. That was challenged, so the TPC took another look; it turns out that even if you throw those into the mix, the richest taxpayers would still get a break.

What’s most disturbing about their claims is that Mr. Romney and his team are unwilling to lay out their plan to back them up. Mr. Hubbard is right that President Obama hasn’t presented a satisfactory plan; we’ve said many times that just raising taxes on the wealthy can’t solve the nation’s fiscal challenge. But Mr. Romney, while specific about the tax cuts he favors, declines to explain which tax expenditures he would close.

“Mr. Romney’s tax reform plan exists,” says Mr. Hubbard. Great: Tell us what it is. Just repeating that it’s possible, without explaining how, isn’t much of an argument.