Mitt Romney is facing renewed pressure to release further years of tax returns following reports on his complicated personal finances and disclosure filings that show he may have been more involved in Bain Capital following his 1999 departure than he has let on.

"Mitt Romney is the most secretive candidate we've seen since Richard Nixon. Mitt Romney provided John McCain 23 years worth of returns when he was being vetted for vice president but he's only shared with the American people one year worth of returns," said Obama campaign spokesman Ben LaBolt on Fox News Friday.

Another Obama campaign surrogate, Maryland Gov. Martin O'Malley (D), made the point more strongly in a Buzzfeed interview by suggesting that Sen. McCain (R-Ariz.) may have picked then-Alaska Gov. Sarah Palin (R) after seeing Romney's returns. The Huffington Post reported that Romney was knocked off the short list because the number of houses that he and McCain owned was too great between them.

And not just the Obama campaign is saying that the tax return question is fair game. Republican Rep. Pete Sessions (Texas) called the tax returns question "legitimate" but declined to say how many years Romney should release. Former RNC Chair and Mississippi Gov. Haley Barbour said on CNN Thursday that he would release more years if he was in Romney's position, but added that it wasn't an issue in the campaign.

After intense pressure during the GOP primary, Romney, who has an estimated net worth of up to $250 million, released a joint 2010 return for himself and his wife in January showing a 13.9 percent effective rate -- much lower than that of most middle-class earners, thanks, in part, to the fact that he derives most of his income from investments. He has filed an extension for 2011 and said recently on Fox Business that he will release the 2011 return when it's ready, adding that there's "nothing hidden" in them.

Then-candidate Barack Obama released his tax returns dating back to 2000 during the 2008 campaign, and McCain released his 2006 and 2007 returns (but not his wife's separate return -- she is far more wealthy than her husband).

The speculation about Romney's reticence to release previous years' returns suggests mostly that there could be something embarrassing -- but not illegal -- in them. At very least, more returns would be another disclosure that sets him apart from the middle-class voters he seeks to champion. Vanity Fair and the Associated Press have investigated Romney's personal finances and found complicated offshore accounts in Bermuda and the Cayman Islands. The Romney campaign says that he derived no tax advantage from them, but without returns, it's an assertion that's impossible to check.

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