Harvard economist Larry Summers says that Mitt Romney's tax plan fails the basic test of arithmetic.

"It's just not possible to do what he says," Summers, who served as a top economic adviser to President Barack Obama, told CNNMoney in a recent interview.

"The reality is that every expert who's looked at it has found that cutting taxes by 20 percent costs $5 trillion," Summers said. "If you take away every exemption, every tax expenditure for people with high incomes, you don't get anything like $5 trillion."

Romney has promised to cut marginal tax rates by 20 percent and slash taxes on investment income without raising taxes on the middle class or increasing the deficit. The Tax Policy Center analyzed his plan in August and concluded that it was mathematically impossible; several commentators agree.

Romney modified his tax plan during the presidential debate on Tuesday, when he said that he would consider capping deductions at $25,000 per household. But the Tax Policy Center found that Romney's tax plan, even with that change, still would increase the deficit by $3.7 trillion over the next 10 years.

Summers criticized another aspect of Romney's economic plan in the interview, saying his campaign's forecast of 4 percent annual GDP growth if Romney is elected is unrealistic.

"Certainly Gov. Romney hasn't suggested anything that would drive the economy forward to the extent where you could be confident that that forecast is realized," Summers said.

GDP grew at an annual rate of 1.3 percent in the second quarter of this year, according to the Commerce Department.

Earlier on HuffPost:

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  • Paul Krugman

    <a href="http://krugman.blogs.nytimes.com/2012/08/10/culture-of-fraud/">The Nobel Prize-winning economist wrote</a> in a New York Times blog post in August: "Romney’s tax plan is now a demonstrated fraud — big tax cuts for the rich that he claims would be offset by closing loopholes, but the Tax Policy Center has demonstrated that the arithmetic can’t possibly work."

  • Matt Taibbi

    Matt Taibbi, contributing editor to Rolling Stone, <a href="http://www.rollingstone.com/politics/blogs/taibblog/the-vice-presidential-debate-joe-biden-was-right-to-laugh-20121012">wrote in a recent blog post </a>: "If you're going to offer an across-the-board 20 percent tax cut without explaining how it's getting paid for, hell, why stop there? Why not just offer everyone over 18 a 1965 Mustang? Why not promise every child a Zagnut and an Xbox, or compatible mates for every lonely single person?"

  • Larry Summers

    Harvard economist Larry Summers, a former top adviser to President Barack Obama, <a href="http://www.huffingtonpost.com/2012/10/11/larry-summers-romney-tax-plan_n_1958982.html">recently compared Mitt Romney's tax plan</a> to a hamburger and ice cream diet. He said: "It’s easy to say that 'My plan is to eat ice cream sundaes and chocolate cake and hamburgers as much as I want, my plan is to lose 60 pounds, and my plan is to avoid painful exercise, and those are all my objectives and I'm committed to every one of them.'"

  • The Tax Policy Center

    <a href="http://www.taxpolicycenter.org/publications/url.cfm?ID=1001628">The Tax Policy Center</a>, a nonpartisan, nonprofit think tank, <a href="http://www.huffingtonpost.com/2012/09/11/romney-tax-plan-middle-class_n_1874113.html">recently concluded</a> that Mitt Romney's tax plan is mathematically impossible without raising taxes on the middle class.

  • Josh Barro

    <a href="http://www.bloomberg.com/news/2012-10-12/the-final-word-on-mitt-romney-s-tax-plan.html">Bloomberg View columnist Josh Barro wrote</a> in a recent column that the six studies that the Romney campaign uses to claim the candidate's tax plan is mathematically possible "individually and collectively...fail the task."

  • Mark Zandi

    <a href="http://thinkprogress.org/economy/2012/10/12/1004921/zandi-romney-tax-plan/">Mark Zandi</a>, chief economist at Moody's Analytics, recently said on CNN that when it comes to Romney's tax plan, "the arithmetic doesn't work as it is right now."

  • Ezra Klein

    <a href="http://www.washingtonpost.com/blogs/ezra-klein/wp/2012/08/04/romney-tax-plan-on-table-debt-collapses-table/">Washington Post columnist Ezra Klein wrote in August</a> that "the Tax Policy Center’s analysis has removed all doubt" that Romney's tax plan is mathematically impossible.

  • David Frum

    <a href="http://www.thedailybeast.com/articles/2012/10/11/vice-presidential-debate-live-blog.html">David Frum</a>, contributing editor at Newsweek and The Daily Beast, recently wrote: "Romney's tax cut plan doesn't work. I'm a Republican, I support Romney, etc. But you can't cut that much in such a stagnant economy and expect to break even. Even with a deductions cap, it just won't happen."

  • Catherine Rampell

    Catherine Rampell, economics reporter at The New York Times, wrote of the <a href="http://economix.blogs.nytimes.com/2012/10/01/the-math-on-the-romney-ryan-tax-plan/">the Romney campaign's tax promises</a> in a recent blog post: "Not <em>all</em> of those principles can coexist so long as basic arithmetic survives."