WE’VE WRITTEN a couple of times about the incomplete nature of Mitt Romney’s tax reform proposal. The man nominated Tuesday as the Republican candidate for president has been specific about all the taxes he wants to abolish and reduce. He’s a lot more vague about how he would keep this from bankrupting the Treasury.

His easy answer is he would “broaden the base” of taxable revenue. This sounds easy — plug an oil company loophole here, shut down a hedge fund dodge there. As Sen. Rob Portman, an Ohio Republican and Romney supporter, told reporters at the Republican convention on Tuesday, “It would be paid for by getting rid of a lot of the underbrush in the code.” Clear out the underbrush! Who could oppose that?

Unfortunately, there is no way to broaden the individual income tax base without going after the biggest tax deductions and preferences, which are widely used, wildly popular and defended by tenacious lobbies. They privilege employer-provided health insurance, mortgage interest, charitable deductions and state and local income tax payments. Mr. Romney hasn’t hinted which of these he might seek to limit.

Mr. Portman, one of the Republicans’ leading budget experts and an advocate of bipartisan fiscal reform, was more forthcoming at a forum sponsored by the Post and Bloomberg News. He suggested that one way to lower rates and yet not see revenue fall off a cliff would be to limit the total that any taxpayer could deduct, without eliminating any specific deduction. “And if you choose to give more to charities and want your charitable deduction, that’s fine,” Mr. Portman said. “But then maybe your second home, you can’t get a mortgage deduction.”

Whether such a plan would mollify political opposition, because every deduction would be saved to some extent, or unify opposition because every one would be threatened is an interesting question. President Obama has repeatedly proposed a similar idea, for upper-income taxpayers, and Congress has repeatedly shown zero interest. And the limits might have to be very constricting — less benign than Mr. Portman’s sunny description — if they were to make up for the revenue lost by Mr. Romney’s tax-cutting proposals.

At least, though, it’s a step toward a substantive proposal. Unfortunately, Mr. Portman stressed that he was speaking for himself in floating it, not for the campaign. “I think what he’ll have when he’s elected — and I believe he’ll be elected — is he’ll have a mandate to do tax reform,” Mr. Portman said. “And then you go to Congress . . .

But how much of a mandate can it be if the candidate talks during the campaign only about the goodies he will deliver, and not at all about the accompanying pain? Given his position at the head of a party dominated by officeholders opposed to tax increases under any circumstances, it’s not an unfair question.