But Diamond also said that the conclusions paint an incomplete picture of the benefits that lower-income Americans would receive from the Romney plan, such as greater employment opportunities and lower lifetime tax burdens.

"If we're going to make all of our policy decisions based off simple analysis like that," he said, "I think that's bad for America."

Diamond has built a body of research advancing the theory that simplifying the tax code - eliminating tax breaks and reducing marginal rates - will unleash growth, by cutting down on economic distortions in the code. About a month ago, the Romney campaign asked Diamond to apply his analytical model to Romney's tax plan, to estimate its job-creation potential.

Romney wants to cut income tax rates by 20 percent across the board. He wants to make the cuts "revenue neutral," eliminating enough tax breaks to offset the lost revenues from lowered rates. He and the campaign have refused to say thus far which loopholes he'd close, but Romney insists that his reform would maintain the current progressivity of the tax code - meaning that a tax cut for the wealthy would not be offset by higher taxes for the poor and middle class.

It is that progressivity goal that three economists at the Tax Policy Center, including one who served on President Obama's Council of Economic Advisers and one who served on the council under Republican President George H.W. Bush, judged earlier this month to be more or less impossible to reach under Romney's plan. Even allowing for enhanced growth from reforming the code, they wrote, there simply are not enough tax exemptions for the wealthy available for elimination to finance lower rates at the top.

As a result, the economists concluded, lower-income taxpayers would see their tax bills rise, on average.

On Friday, Romney ripped the Tax Policy Center study and touted Diamond's analysis of his proposal. "Rice University looked at my plan," Romney said, "and said, 'Mitt Romney's tax plan will create millions of jobs.' "

This is where things get tricky, economically and politically.

Diamond is not an adviser to the Romney campaign. He couldn't model the full Romney plan, because the Republican's campaign refuses to identify, even to him, which tax deductions and credits they would cut to make the plan revenue-neutral. "They didn't give me a whole lot of guidance," he said.

So Diamond filled in the blanks with largely generic assumptions and noted in his analysis that the specifics could change his results. He also assumed that Romney - contrary to his stated campaign position - would allow the Bush tax cuts to expire and then impose his 20 percent across the board rate reduction from there.

Diamond's final conclusion was that Romney's reform would add 5.4 percentage points to projected gross domestic product growth over the next decade and would result in 6.8 million new jobs.