With only four months left until the election, the Obama campaign has decided to attack Mitt Romney's past rather than his platform.

In an interview with CBS Radio News on Thursday, veteran Obama adviser David Axelrod accused Romney of "trying to turn back the clock" on transparency by 40 years, and called him "the most secretive candidate we've seen probably since Richard Nixon."

During a campaign stop in Parma, Ohio, Axelrod challenged the GOP presidential candidate to release a complete set of tax returns as well as records about any other undisclosed assets, particularly accounts in Bermuda, the Cayman Islands and Switzerland.

"We don't know what the nature of all these accounts are overseas and [Romney] really won't provide much information about them," Axelrod said, "so you'd have to ask him if he's hiding something."

Axelrod repeated this talking point during an interview with ABC News Radio.

Romney "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.

According to The Associated Press, the former governor of Massachusetts is worth up to $250 million, but an exact accounting of his financial background has never been released. During the GOP primary in 2007, Romney was criticized by his rivals because some of his wealth reportedly came from investments in corporations with interests in Iran, China and embryonic stem cell research. When this lack of transparency became fodder for debate during the latest GOP primary, Romney released his 2010 tax return. He has declined to release returns from earlier years.

In response to Axelrod's comments, the Romney camp accused the Obama "political machine" of trying to refocus the campaign away from the struggling economy.

"With millions of Americans suffering in the Obama economy, it's no surprise that President Obama and his allies would resort to false, negative attacks in an attempt to distract Americans from the President's abysmal record," campaign spokeswoman Amanda Henneberg told CBS.

LISTEN TO AXELROD'S INTERVIEW BELOW:

video platformvideo managementvideo solutionsvideo player

Also on HuffPost:

Loading Slideshow...
  • Chicago Mayor Rahm Emanuel

  • Massachusetts Gov. Deval Patrick

  • Montana Gov. Brian Schweitzer

  • Former Pennsylvania Gov. Ed Rendell

  • Newark Mayor Cory Booker

  • N.Y. Gov. Andrew Cuomo

  • Sen. Jon Tester (D-Mont.)

  • L.A. Mayor Antonio Villaraigosa

  • Sen. Jim Webb (D-Va.)

  • Former President Bill Clinton