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.