WASHINGTON, Oct 17 (Reuters) - Mitt Romney's proposed cap on itemizing tax deductions could not on its own raise enough new government tax revenue to compensate for revenues lost by the Republican presidential candidate's plan to slash income tax rates, a think tank said on Wednesday.

The Tax Policy Center, a nonpartisan group that has weighed in on other Romney proposals, said his deductions cap could raise up to $1.7 trillion over 10 years. The center said earlier this year Romney's 20-percent tax rate cut would cost $4.8 trillion.

The former Massachusetts governor has argued that his plan will not cost $4.8 trillion. At a debate on Tuesday with Democratic President Barack Obama, Romney reiterated that he would pay for his tax cut proposal by capping tax deductions by a set dollar amount. Taxpayers could choose their deductions under the cap, such as the home mortgage interest and charitable donation write-offs, among others, he said.

"I'm going to bring rates down across the board for everybody, but I'm going to limit deductions and exemptions and credits, particularly for people at the high end," Romney said at the debate in Hempstead, New York.

The Tax Policy Center acknowledged its latest estimates were based on an incomplete picture of Romney's tax plan.

"The Tax Policy Center has again inserted their own assumptions in order to reach a biased conclusion," a Romney campaign spokeswoman said on Wednesday.

The Romney campaign had previously criticized the Tax Policy Center's estimates, saying they did not account for economic growth that can pay for tax cuts and that the center excluded some tax breaks in their studies.

The campaign has said the limit on itemized deductions would be only part of its plan to fund the rate cut. For instance, it would also revamp the tax treatment of healthcare, which now comes in the form of an exclusion when health insurance is workplace-based.

Romney has shifted the dollar amount taxpayers might be able to deduct. "I'll pick a number - $25,000 of deductions and credits, and you can decide which ones to use," he said.

Romney earlier this month floated a cap on deductions set at $17,000. His campaign later said that proposal is one of a range of options. Romney has also said $50,000 could serve as the cap.

The higher the cap, the less money Romney's tax plan could raise to offset tax rate cuts, the center's estimates show.

A cap of $17,000 would raise $1.7 trillion over 10 years while the $50,000 cap would raise only $760 billion. If Romney eliminated all itemized deductions, his plan could raise $2 trillion over 10 years, the center has estimated.

Obama has called for a cap on itemized deductions of 28 percent of adjusted gross income for individuals earning more than $200,000 a year and families earning more than $250,000.

Copyright 2012 Thomson Reuters. Click for Restrictions.

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  • Ron Paul

    "Politically, I think that would help him," Paul said in a interview with <a href="http://www.politico.com/news/stories/0712/78599.html#ixzz20w7ycTBE" target="_hplink">Politico</a>. "In the scheme of things politically, you know, it looks like releasing tax returns is what the people want."

  • Richard Lugar

    "I have no idea on why he has restricted the number to this point," <a href="http://www.cbsnews.com/8301-18563_162-57474352/pressure-mounts-on-mitt-romney-to-release-more-tax-returns/?tag=socsh" target="_hplink">Lugar said</a>.

  • George Will

    "I don't know why... he didn't get all of this out and tidy up some of his offshore accounts and all the rest," Will said on ABC's "<a href="http://www.huffingtonpost.com/2012/07/15/george-will-mitt-romney-bain_n_1674513.html?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+HP%2FPolitics+(Politics+on+The+Huffington+Post)" target="_hplink">This Week</a>." "He's done nothing illegal, nothing unseemly, nothing improper, but lots that's impolitic."

  • Bill Kristol

    "He should release the tax returns tomorrow. It's crazy," Kristol said on "<a href="http://www.huffingtonpost.com/2012/07/15/mitt-romney-tax-returns_n_1674325.html" target="_hplink">Fox News Sunday</a>." "You gotta release six, eight, 10 years of back tax returns. Take the hit for a day or two."

  • Robert Bentley

    "I just believe in total transparency," Bentley told <a href="http://abcnews.go.com/blogs/politics/2012/07/gop-governor-calls-on-mitt-romney-to-release-additional-tax-returns-and-show-he-has-nothing-to-hide/" target="_hplink">ABC News</a>. "In fact, I was asked today that question -- do you think that Governor Romney should release his tax returns? And I said I do. I said, I release my tax returns. I may be the only public official in Alabama that does, but I release mine every year and I just believe that people should release their tax returns. And if you get them out and just get past that, it just makes it so much easier."

  • Haley Barbour

    When asked on "<a href="http://www.nationaljournal.com/2012-presidential-campaign/barbour-romney-should-release-more-tax-returns-20120710" target="_hplink">The Situation Room</a>" if Romney should release more returns, Barbour said, "I would. But should it be an issue in the campaign? I don't think it amounts to diddly."

  • Michael Steele

    "If there's nothing there, there's no 'there' there, don't create a there,'" Steele said on MSNBC.

  • David Frum

    "Tax returns the next problem. Releasing returns under pressure: more weakness, more pain," Frum <a href="https://twitter.com/davidfrum/status/224509505973661696" target="_hplink">tweeted</a>.