Perhaps no Republican has embraced one of the central talking points of GOP tax policy -- that lower taxes actually can bring higher revenues -- more than Rudy Giuliani.

Giuliani has made the argument as recently as this month's debate in Des Moines, Iowa, and has aired advertisements that clearly make such a claim: "There's no question -- taxes go down, revenue goes up," Giuliani says in a radio ad that has aired in Iowa and New Hampshire. "I know that reducing taxes produces more revenue."

But a broad consensus among economists, both liberal and conservative, suggests the claim is simply not true. Though Republicans since Ronald Reagan have argued that tax cuts spur economic growth that actually can increase government income, economic models strongly suggest that such effects replace a relatively small portion of the income the government loses when tax rates go down.

Michael Boskin, Giuliani's top economic adviser, denies that the former New York mayor has overstated the effects of tax cuts. Giuliani's seemingly uncategorical declaration is "shorthand," Boskin said, for a much more limited claim -- that economic growth stimulated by cuts always will replace some of the lost revenue, and in some situations all of it, depending on the specific circumstances.

"That's the mayor's view and that's exactly correct," said Boskin, an economist a Stanford University and a former head of the Council of Economic Advisers under the first President Bush. "What he's saying is that sometimes they do (replace all the lost revenue); you'll always get some of it back and you'll help the economy."

It's an argument that the current President Bush has made often in justifying his own income tax cuts. And it has a basis in fact: Economists generally agree that, all things being equal, tax cuts do boost the economy. That stronger economy creates a bigger revenue pie, so that the government, even getting a smaller slice of that bigger pie, does not lose revenue on a one-for-one basis with the size of the tax cut. Economists use terms such as "feedback," "dynamic effects" or "supply-side effects" to describe the theory.

But a wide variety of economists -- including conservatives such as the American Enterprise Institute's Alan Viard and Gregory Mankiw, a former Bush adviser who advises Mitt Romney's campaign -- conclude the revenue increase generally doesn't make up for all of the tax cut. Analysis by government agencies such as the Congressional Budget Office, the Office of Management and Budget and the Treasury Department all show that, regardless of economic assumptions, the Bush tax cuts have lowered government revenues.

"It's a convenient political argument with no factual basis," said Robert Bixby, head of the Concord Coalition, which advocates for a balanced budget. The supply-side argument allows politicians to advocate tax cuts without making the difficult spending cuts to offset them, Bixby said.

Other Republican candidates have made similar claims. Mitt Romney, in a speech this March to the anti-tax Club for Growth, repeated the claim. John McCain has said in interviews that the Bush cuts helped boost government revenue. But they have appeared to back off that argument, at least publicly.

"It's a complicated question," said Romney adviser James Bognet. "The amount of feedback you get entirely depends on the kind of tax. I don't think Governor Romney would stand up there and tell you what percentage of feedback there's going to be."