Republican Senator Marco Rubio of Florida visited Bloomberg View today, where he displayed the charm, intellect and communication skills that have marked him as a rising star. Despite a wealth of political talent, however, there seemed little Rubio could do to make sense of the Mitt Romney campaign's tax plan.

Asked which exemptions he personally would eliminate from the tax code in order to accommodate Romney's 20 percent across-the-board tax cut, Rubio instead spoke of the loopholes he would safeguard. They happen to be big ones: the home mortgage deduction, the charitable-giving deduction and the exclusion for health insurance. The mortgage interest deduction alone costs the U.S. Treasury almost $100 billion annually.

Romney's whole plan is premised on the notion that loopholes would be eliminated in return for the lower tax rates. This, plus a near-magical dollop of economic growth, would lead to balanced budgets. Except there aren't enough loopholes in the tax code to compensate for the tax cuts and there is little reason to suspect growth will rise sky-high on the strength of Romney's hot air. And even Romney says he doesn't want to do away with popular -- and costly -- tax deductions.

In response to a question from Wolf Blitzer, Romney stated:

With regards to the deductions you describe, home mortgage interest deduction and charitable contributions, there will of course continue to be preferences for those types of expenses.

A vague commitment to continuing "preferences for those types of expenses" is hardly a commitment to anything at all. But if Romney seems squishy about details, it's not because he doesn't know how to communicate about numbers. It's that the plan is exceedingly difficult to defend once specifics enter into the conversation.

In today's meeting, Rubio was impressive on a broad array of topics, especially immigration. When it came to defending Romney's tax plan, however, he was as lost as Romney.

(Francis Wilkinson is a member of the Bloomberg View editorial board. Follow him on Twitter. He will live-blog tonight's presidential debate at Bloomberg's politics section.)

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