When the Romney slime, and the Romney Super-Pac slime, start to hit Rick Santorum, he’s going to need some responses. And the best defense is a good offense.

So here, in the spirit of “love your enemies, and bless them that curse you,” is some free advice for the Santorum campaign: ask Mitt Romney whether the $3 million Swiss bank account revealed by his 2010 taxes but not disclosed in his previous financial statements – had been declared on previous tax returns. The alternative is that it was declared only in 2010, and only in response to the IRS FBAR amnesty program for taxpayers who had illegally neglected to mention their holdings at UBS and other havens of bank secrecy. That program was started after the government squeezed thousands of names of U.S. tax cheats out of UBS.

It’s not a hard question to answer, if you have access to the 2009 return, which Romney continues to withhold. So it needs to be asked directly, and persistently.

Yes, it’s possible that there was an innocent reason for having $3 million stashed in an account held by one of the leaders of the Swiss tax-evasion industry. But “diversification,” without more, is hardly adequate.

This is a no-lose proposition for Santorum. Reminding voters that he’s running against the sort of guy who can leave $3m in a low-interest account for no apparent purpose is good; if the purpose was really tax evasion, that’s even better.

Just asking the question, “Did you participate in an IRS amnesty program for tax cheats?” is a win; as LBJ said, even if you can’t prove it, you can make the SOB deny it. And no one is going to pay much attention Romney’s whining about unfairness after his “Pants on Fire” attempt to put a McCain adviser’s words in Barack Obama’s mouth.

Live by the slime, die by the slime.

[Cross-posted at The Reality-Based Community]

Mark Kleiman

Mark Kleiman is a professor of public policy at the New York University Marron Institute.