Updated at 10:05 a.m. ET
(CBS News) As Mitt Romney travels to Pittsburgh today, the Obama campaign is aiming to sow new doubt into the minds of Pennsylvania's swing voters as to whether the Republican presidential candidate is paying his fair share in taxes.
In a new ad airing in the battleground state, President Obama's team takes its latest swing at Romney for .
"Romney admits that over the last two years he's paid less than 15 percent in taxes on $43 million in income," a narrator says in the ad. Then, as the years 2009, 2008, 2007, 2006 and 2005 flash on the screen, the narrator continues, "Makes you wonder if some years he paid any taxes at all."
The ad charges Romney with using "every trick in the book," such as tax havens and offshore accounts, to avoid paying taxes and ends by asking, "What is Mitt Romney hiding?"
In response to the ad, the Romney campaign charged that Mr. Obama is the one with transparency issues.
(Watch CBS News' Bill Plante's report on attacks heating up between Obama and Romney.)
"Whether hiding lobbyists in coffee shops, cutting back-room deals on Obamacare, or concealing the records of 'Fast and Furious,' President Obama's pledge to be transparent has turned out to be just another broken promise," Romney campaign spokesperson Amanda Henneberg said in a statement to CBS News.
Romney today is attending a fundraiser in Pittsburgh, as well as a rally in Irwin, a borough just outside of Pittsburgh. He's also taping a series of interviews with local Pittsburgh news outlets. The southwestern region of Pennsylvania could prove to be a critical area in this year's presidential election.
Mr. Obama won Pennsylvania by more than 620,000 votes in the 2008 election, but his success was based on a strong showing in and around Philadelphia. But as CBSNews.com's Brian Montopoli, the president this year is facing a drop in enthusiasm in Philadelphia. If he wants to win the battleground state again, he will have to perform well in areas around Pittsburgh - including places he lost in 2008 like Beaver County.
The region is largely comprised of working class, union-friendly white voters who may be amenable to the Obama campaign's argument that Romney isn't paying his fair share in taxes.
And ashows that the issue of releasing tax returns has been a potent one for decades. Multiple candidates have been dogged by the issue, including one eventual president - Ronald Reagan - who, it turns out, avoided paying state taxes one year.
While Romney is in Pennsylvania today, Mr. Obama flies down to Texas for a series of fundraisers from which he's expected to raise a total of at least $4 million. The president in June raised $71 million, compared to Romney's $106 million in the same month, making June the second month in which Romney outraised him.
This story was corrected to note that President Obama is expected to raise a total of at least $4 million today in Texas, not $5.9 million.