"[Reducing the mortgage interest deduction] is troubling, because it really helps the middle class," he said. "Do you really want to hurt charitable giving in a country when you are saying that you want to rely less on government and more on private institutions to deal with these issues? And how are you going to raise taxes on people on their health care premiums when you are saying you want there to be a system in place where folks can have more control over their own money?"
The tax exclusion on employer-provided health care and mortgage interest deduction are the two largest expenditures in the federal tax code. Charitable giving ranks in the top 10. All three, among others, are popular and have strong bipartisan support in Congress.
That a leading surrogate would be reluctant to roll them back makes Romney's tax math even less realistic -- and thus more reliant on dubious conservative tax cut assumptions.
"Here is where you get into that debate," Rubio said. "You don't dynamically score these things ... I know you are going to roll your eyes. But I am a firm believer that economic growth is generated by these things and the evidence is there."
The most comprehensive analysis of Romney's tax plan, performed by the nonpartisan Tax Policy Center, borrowed economic growth assumptions from one of Romney's own advisers. Despite those assumptions, they concluded Romney's plan would either require higher taxes on middle-income earners or tolerating larger budget deficits.
Rubio's remarks that he'd want Romney to tread lightly around big, popular deductions suggest that Romney's across the board 20 percent tax rate cuts could simply end up starving the federal government of revenue.
A Rubio spokesman did not immediately respond to TPM's request for comment.