Romney is touring those swing states by bus starting Saturday.
"Did Romney pay 10 percent in taxes? 5 percent? Zero? We don't know,” the voiceover on the new ad says.
The suggestion that Romney might have paid zero percent of his income in taxes seems to be a reference to accusations made by Senate Majority Leader Harry Reid (D-Nev.) that Romney avoided paying taxes for a decade. Reid has cited an anonymous investor from Romney’s former private-equity firm Bain Capital for the claim.
The ad goes one step further in suggesting Romney also helped corporations he managed in his previous career to dodge taxes. It seeks to tie Romney to a tax shelter scheme known as "Son of BOSS” through Romney’s previous involvement with Marriott Corp., one of the companies implicated in the scheme in the '90s.
The Romney campaign called the ad "another dishonorable and dishonest attack."
"Since President Obama can’t run on record unemployment, falling incomes, and massive debt, he has decided to run a dirty campaign that is an affront to everything he claims to stand for," Romney spokeswoman Amanda Henneberg said.
The Obama campaign is pushing Romney to release more tax returns in order to answer questions they continue to raise about what is in them. Romney has held firm that his decision to voluntarily release two years of tax returns is enough.
He told Bloomberg Businessweek in an interview published Thursday that he has met and gone beyond all financial disclosure requirements for presidential candidates.
"I’m not a business,” he said, when asked if voters deserved a longer look at the history of the “investment” they are making in a future president.
“Apparently, corporations are no longer people in Romney’s mind,” Obama for America Press Secretary Ben LaBolt said in a memo, referring to a comment Romney made last year.
The Democratic National Committee will also be following Romney on the campaign trail, in a “Romney Economics” bus painted with warnings that Romney will throw the middle class under the bus as president.
Updated at 5:43 p.m.