A few weeks ago, Paul Begala, one of the operatives behind the Obama-allied group Priorities USA Action, emailed me an offhand gag:

The “Romney Rule” seems to be that millionaires like Mitt should pay a lower tax rate than maids.

Now it looks like the “Romney Rule” is making its official debut in Campaign 2012. Priorities USA today released a new video on the “Romney Rule.” The video is an attack on Romney, who derives much of his income from investments, for paying a lower tax rate than many middle class taxpayers:

“Have you heard of the Romney rule? It’s pretty simple,” the video says. “Mitt Romney believes millionaires like him are entitled to pay taxes at a lower rate than teachers, cops, or construction workers.”

As I’ve been noting here regularly, Dems are currently experimenting with how far to go in attacking Romney over his wealth and tax rate. The Obama campaign itself has not been willing to go here yet, preferring instead to leave this attack line to the DNC and outside allies.

But I continue to think that Romney’s tax rate in particular remains one of his unexplored vulnerabilities in a general election. Fair taxation is going to be one of the major issues in Campaign 2012. Obama’s push for the closure of tax loopholes, the call for a “Buffett Rule” targeted at those millionaires who pay lower tax rates than middle class taxpayers, and the coming argument over the expiration of the Bush tax cuts for the rich will all coincide to ensure that this topic is central next year.

You’d think the fact that Romney himself is one of the “millionaires and billionaires” who pays lower tax rates would render him a less than ideal messenger to make the case against Obama’s push for tax fairness. After all, Obama — or his outside allies — can respond by pointing out that Romney himself personally profits from the system Obama is trying to reform. Indeed, that’s what they’re now officially doing.

Republicans will argue that Obama risks looking like too much of a class warrior who’s looking to penalize wealth and success if he runs with this line. And indeed, his campaign has not yet picked it up yet. But Dems view this as a way to exploit Romney’s vulnerabilities without being tarred as anti-success: They are merely arguing that Romney should pay the same tax rate as middle class taxpayers.

Either way, Romney’s millionaire tax rate is going to be more central to the campaign than I think many realize.