By Steve Holland and Patrick Temple-West
CHANDLER, Ariz./WASHINGTON (Reuters) - Republican Mitt Romney revised his proposal for overhauling the U.S. tax code on Wednesday, calling for all individual tax rates to be cut by 20 percent while declining to offer specifics on how to make up the lost revenue from lower rates.
Romney's new tax plan would put the top individual tax rate at 28 percent, down from the top statutory 35 percent rate and matching rival Republican presidential candidate Rick Santorum's proposed top rate.
Romney is seeking to regain momentum in his campaign for the 2012 Republican presidential nomination to face Democratic President Barack Obama in the November 6 election. The former Massachusetts governor trails Santorum in some national polls ahead of election primaries in Arizona and Michigan on February 28.
On Wednesday night, Romney, Santorum and the two other Republican candidates, Newt Gingrich and Ron Paul, hold a debate in Arizona. Romney is scheduled to give an economic speech on Friday in Detroit.
"The right way forward is a flatter, fairer, simpler tax system that generates the revenue we need to fund a smaller government," Romney said during a campaign rally in Chandler.
The Romney campaign said on Wednesday that the candidate's overall budget plan would be revenue neutral. But it provided no details on which politically contentious tax breaks would be cut to avoid ballooning the budget deficit if his tax cuts were implemented.
Romney campaign officials held a conference call with at least five tax think tank groups on Wednesday.
The campaign officials told participants that the tax breaks for mortgage interest, charitable giving and retirement savings would be preserved. Decisions on other tax breaks would not be made during the campaign, they said.
"Romney understands that his economic plan was uninspiring up until this point and he needed to step it up in order to broaden his appeal," said Scott Hodge, president of the nonpartisan business-oriented Tax Foundation, who was not on the call.
"This really falls short of the sweeping tax reform plan that I think a lot of the grass roots seem to be waiting for," he said.
Romney's personal tax rates would not be affected under his own plan, said Bob Williams, a senior fellow at the nonpartisan Tax Policy Center, who compared Romney's tax plan to the candidate's own tax returns.
When he released his 2010 tax returns in January, Romney drew fire for his 13.9 percent effective tax rate on more than $21 million in income. Most of that was investment income, which under the U.S. tax code is taxed at a lower rate - 15 percent - than what many wage-earning Americans pay.
Romney's plan would make permanent the 15 percent tax on investment income.
Romney's estate would benefit from eliminating the estate tax, which his plan calls for. He also could gain indirectly from a lower business rate if companies pay more in dividends.
"The tax return we all saw and that people were bemoaning for the 13.9 percent tax rate, that wouldn't change," Williams said.
Previously, Romney had proposed making permanent the Bush-era tax cuts, which have a top 35 percent tax rate.
(Editing by Howard Goller and Mohammad Zargham)