With Mitt Romney still refusing to release his tax returns, we can only speculate as to why he’s so afraid of disclosure. The widely held assumption has been that the Republican frontrunner is embarrassed by his “15% problem” — the multi-millionaire takes advantage of existing tax loopholes to pay lower tax rates than millions of middle-class workers.

But that’s not the only thing Romney’s returns might show. Reuters’ Sam Youngman reported on a related issue:

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.

Greg Sargent followed up with Berman, who added this may help explain Romney’s intention to maybe release one year’s returns, but keep previous years secret.

“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.”

It’s worth clarifying that no one has accused Romney of doing anything illegal. Plenty of very wealthy Americans take advantage of offshore strategies. That’s not a crime.

For that matter, without more information, we don’t know what kind of tax benefit, if any, Romney enjoyed from offshore accounts.

But it doesn’t exactly help paint a picture of Romney as a “man of the people,” and it serves as a reminder of the importance of the tax returns the former governor is so eager to keep hidden. It now appears we have a Republican frontrunner who amassed a vast fortune after laying off thousands of American workers, wants to give himself another tax cut, owns multiple luxury homes, and stashes cash in the Caymans — all while pursuing an agenda that would make things tougher on American’s working class.

It’s not surprising, then, that the National Review wants those returns immediately: “It is critical that Romney release his tax records now, that voters might ‘take a look and decide if we’ve got a flawed candidate. We know that should Romney become the nominee, he will be criticized over the sources of his wealth and will have to effectively respond. Republican primary voters deserve to see whether he can do so before they vote.”

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Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.