Why has Mitt Romney been reluctant to release his tax returns? Well, there’s the small matter of how much he may benefit from the “carried interest” loophole that private equity managers enjoy.

But Reuters’s Sam Youngman talks to tax experts who suggest another reason — they could contain evidence that Romney sheltered income offshore:

Tax analysts say Romney may have good reason to be reluctant to release his returns.

His vast fortune is invested in dozens of funds linked to Bain Capital LLC, the powerhouse private equity firm he co-founded and led for 15 years. Several Bain funds have offshore connections and take advantage of tax breaks used only by the U.S. financial elite.

His tax returns could shed light on how Romney and Bain use offshore strategies to avoid taxes, said Daniel Berman, a former U.S. Treasury deputy international tax counsel and now director of tax at Boston University’s graduate tax program.

Bain funds in which Romney is invested are scattered from Delaware to the Cayman Islands and Bermuda, Ireland and Hong Kong, according to a Reuters analysis of securities filings.

“Certain interests in foreign investment structures would have to be reported on attachments to his return,” Berman said.

I’ve checked back with this expert to ask him whether this would be an incentive for Romney not to release tax returns for previous years, and will update you when I hear back. Keep in mind that Romney said yesterday that he was prepared to release his 2011 return — the one that would be filed this year.

As Suzy Khimm notes, offshore shelters would constitute “yet another tax advantage for Romney.”

We obviously don’t know what’s in those returns, but if these analysts are right, this could prove yet another political weapon for Dems. If the Democratic strategy is to paint Romney as the walking embodiment of everything that’s unfair about our tax system and of all the ways the system is rigged on behalf of the rich and against the middle class, this would consitute a pretty potent data point.


UPDATE: I just got off the phone with Berman, the expert quoted above, and he suggested offshoring, if it’s in the returns, could be a disincentive to releasing ones from previous years.

“It is interesting that he is offering to release tax returns that have not yet been filed — he may have anticipated this and done some restructuring of his investments in time for them to be reported on the 2011 return,” Berman said.

“The fact of participating in certain offshore structures would have to be disclosed,” Berman continued. “And it could be that he’s disinclined to release past returns because they very well may disclose that he participated in offshore investment structures.”