Since that conversation, I have asked the same question of conservative economists and received similar answers. “To me, the Bush tax cuts get too much attention,” said R. Glenn Hubbard, who helped design them as the chairman of Mr. Bush’s Council of Economic Advisers and is now a Romney adviser. “The pro-growth elements of the tax cuts were fairly modest in size,” he added, because they also included politically minded cuts like the child tax credit. Phillip L. Swagel, another former Bush aide, said that even a tax cut as large as Mr. Bush’s “doesn’t translate quickly into higher growth.”

Why not? The main economic argument for tax cuts is simple enough. In the short term, they put money in people’s pockets. Longer term, people will presumably work harder if they keep more of the next dollar they earn. They will work more hours or expand their small business. This argument dominates the political debate.

But tax cuts have other effects that receive less attention — and that can slow economic growth. Somebody who cares about hitting a specific income target, like $1 million, might work less hard after receiving a tax cut. And all else equal, tax cuts increase the deficit, as Mr. Bush’s did, which creates other economic problems.

When the top marginal rate was 70 percent or higher, as it was from 1940 to 1980, tax cuts really could make a big difference, notes Donald Marron, director of the highly regarded Tax Policy Center and another former Bush administration official. When the top rate is 35 percent, as it is today, a tax cut packs much less economic punch.

“At the level of taxes we’ve been at the last couple decades and the magnitude of the changes we’ve had, it’s hard to make the argument that tax rates have a big effect on economic growth,” Mr. Marron said. Similarly, a new report from the nonpartisan Congressional Research Service found that, over the past 65 years, changes in the top tax rate “do not appear correlated with economic growth.”

Mr. Romney and Mr. Ryan, to be sure, are not calling for a simple repeat of the Bush tax cuts. They say they favor a complete overhaul of the tax code, reducing tax rates by one-fifth (taking the top rate down to 28 percent) and shrinking various tax breaks. Many economists think such an overhaul could do more good than the Bush tax cuts, by simplifying the tax code.

The problem for anyone trying to evaluate the Romney plan, however, is that there isn’t a full plan yet. He will not say which tax breaks he would reduce, and the large ones, like the mortgage-interest deduction, are all popular. In a painstaking analysis, the Tax Policy Center showed that achieving all of Mr. Romney’s top-line goals — a revenue-neutral overhaul that does not increase the tax burden of the middle class — is not arithmetically possible. History is littered with vague calls for tax reform that went nowhere.