MAUMEE, Ohio - Senior Obama campaign strategist David Axelrod today accused Republican presidential rival Mitt Romney of being "the most secretive candidate that we've seen, frankly, since Richard Nixon."

He "won't release his tax returns, won't reveal his bundlers, left Massachusetts with the hard drives from his computer. He believes in keeping the public in the dark," Axelrod said in an interview with ABC News Radio at the first stop on Obama's campaign bus tour through Ohio.

Axelrod said Romney's portfolio of offshore investments - which include a Bermuda-based company, funds in the Cayman Islands and a Swiss bank account - raises what he called a "Bermuda Triangle" of financial questions.

"Just what were the benefits there? Did he actually pay taxes in some years? We don't know," he said.

"Why would you transfer your Bermuda business…to your wife the day before you became governor? Why did you not want that on your disclosure form?" Axelrod said later, referring to reports by The Associated Press and Vanity Fair.

Asked directly whether he is accusing Romney of tax cheating, Axelrod said no.

"The only reason to have these accounts is to shelter your taxes," he said. "The question is, exactly what did he pay, and I think that he - there's only one way to answer that question. If you don't have anything to hide, why not just release that information? And Gov. Romney really doesn't have a good answer for that."

In January, Romney released his 2010 tax return amid intense public scrutiny and pressure from his GOP primary rivals. He has declined to release returns from additional years.

Asked about Axelrod's comments, Romney campaign spokeswoman Andrea Saul said in an emailed statement that they are "false."

"As job growth slows, manufacturing activity stalls, and our economy continues to sputter, President Obama knows he can't make a legitimate argument for another term in office, so instead he is trying to tear down his opponent," she wrote. "This is just the latest example of President Obama and his political machine saying or doing anything to distract from his abysmal record over the last four years."

ABC News' Devin Dwyer contributed to this report. This post has been updated.

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