In an interview with CBS Radio News on the sidelines of an Obama campaign rally Thursday evening in Parma, Ohio, senior campaign adviser David Axelrod called Romney, "the most secretive candidate we've seen probably since Richard Nixon."
Axelrod accused Romney of "trying to turn back the clock" on transparency by 40 years. The veteran Obama adviser criticized Romney for failing to release "a full set of (tax) returns like most candidates."
(Watch Axelrod's Friday interview on "CBS This Morning" above.)
Axelrod also noted Romney's past use of accounts "in the Cayman Islands, Bermuda, and the Swiss account."
"We don't know what the nature of all these accounts are overseas and he really won't provide much information about them, so you'd have to ask him if he's hiding something," said Axelrod.
He challenged the GOP presidential candidate to "release all these records," adding, "if there are no questions, then it will resolve the whole matter."
The Romney campaign issued a swift response.
"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 Radio News.
Henneberg accused the Obama "political machine" of "saying or doing anything to try to cover up record-high unemployment" and other economic challenges.
The sharp exchange between the two campaigns developed as President Obama wrapped up the first day of a two-day campaign swing through parts of Ohio and Pennsylvania.
Tensions between the two camps have been building. In an interview with CBS News correspondent Jan Crawford, Ann Romney complained that the Obama campaign was "trying to portray him (Mitt Romney) in a light that is just completely wrong."
Axelrod, meanwhile, said Romney has "spent every minute of his waking day in an unalloyed carpet bombing of the president."