The Obama camp aired an ad in the Pittsburgh media market taking issue with Romney's decision to only release two years of his personal tax returns, a full record from 2010 and an estimate for 2011. Obama officials have no evidence supporting the suggestion that Romney may have paid zero taxes in some previous years.
"We won't know, and it's not possible for anyone to know, until he releases further years of tax returns," said Obama campaign spokeswoman Jen Psaki.
In an online editorial, The National Review urged Romney to release additional tax returns even though it agreed with him that the Obama campaign wanted the returns for a "fishing expedition."
"By drawing out the argument over the returns, Romney is playing into the president's hands," the editorial stated. "He should release them, respond to any attacks they bring, and move on."
Romney stepped up his criticism of Obama in Pennsylvania, which has been a tough presidential battleground for the Republican Party. He accused Obama of engaging in cronyism, citing federal grants and loan guarantees to alternative energy companies run by Obama backers and donors.
In choosing Texas to raise money, Obama ventured to a state that has not voted Democratic in a presidential contest since 1976. But Texas ranks among the states with the largest concentrations of wealth, along with New York, California, Florida and Illinois.
Republicans typically raise more in Texas than Democrats. So far this election, Obama has raised $4.5 million from the state and the Democratic National Committee has raised $1.7 million, compared with $7.1 million for Romney and $5.3 million for the Republican Party.
Obama held one event in San Antonio aimed in part at Latinos, featuring actress Eva Longoria. Another event in Austin is co-sponsored by the Democratic National Committee's LGBT Council. He also scheduled two smaller events with high-dollar donors.
Kuhnhenn reported from San Antonio. AP White House Correspondent Ben Feller and Associated Press writers Ken Thomas and Jack Gillum in Washington and Kantele Franko in Columbus, Ohio, contributed to this report.
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