Gregory Mankiw, an economic adviser to Mitt Romney, has penned an Op ed in which he seems to defend the fact that “carried interest” is taxed at a lower rate than ordinary income.

Romney himself, of course, personally benefits to an untold degree from this tax loophole. Meanwhile, Obama has called for a “Buffett Rule” that would end this, and the debate over carried interest will be a major issue in the presidential campaign. Alec MacGillis marvels at the political implications of this:

It is one thing to have a candidate who is committed to promoting unjust tax policies that will help you and your fellow millionaires; it is another thing to have a candidate who benefits from those unjust policies, thus making himself a poster child for reform. Romney pays a federal income tax rate of only 14 percent on his income of more than $20 million per year because of two features of the tax code. First, capital gains on investments, the bulk of his income, are taxed at only 15 percent, well below the top marginal rate of 35 percent for ordinary income. Second, the income he still collects from Bain Capital, as part of his retirement deal there, is eligible for the carried-interest loophole, which allows fund managers to have their compensation for investing other people’s money taxed as capital gains, not earned income... looks like Team Romney will not let his personal stake in the matter keep it from arguing vociferously for maintaining the carried-interest loophole. And for the rest of us, it looks like we better get ready for many comparisons of the plight of Mitt Romney and other multi-millionaire money managers to the plight of a humble carpenter.

This is something that I’ve been marveling at, too. And it’s actually worse than this. It’s very likely that Congress is, in some form or other, going to actually hold a vote on doing away with this loophole. Obama has proposed a “Buffett Rule” as part of corporate tax reform; Senator Sheldon Whitehouse has offered his own version, which has been endorsed by Buffett himself. We don’t know exactly how or when, but members of Congress will be forced to go on record about this.

What that means is that the vast majority of Republicans in the House and Senate will likely cast votes that would have the effect of protecting the enormous wealth of the man at the top of the ticket, whose worth has been estimated at north of $250 million.

No one is saying that Romney has adopted these policies for the express purpose of protecting his own wealth. But that doesn’t mean the politics and optics of this could prove to be absolutely awful. It’s one thing for Romney and Congressional Republicans to defend these policies on principle; it’s another for them to oppose Obama’s proposed changes when they would force him to pony up at middle class rates.

It’s perfectly possible that the economy will trump all and that voters won’t see the issue in these terms. But it’s still hard to see how Romney is the right messenger to make the case against Obama’s push for tax fairness, which will be absolutely central to Campaign 2012. At the very least, having an enormously wealthy presidential candidate defend the idea that he and many members of his class should pay a far lower rate than many middle class taxpayers do should prove an interesting political experiment.