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WASHINGTON - Republicans stepped up their attacks on Senate Majority Leader Harry Reid on Sunday, accusing him of lying by saying Mitt Romney didn't pay taxes for a 10-year period.

Sen. Lindsey Graham, R-S.C., said Reid was "making things up," and Republican National Committee Chairman Reince Priebus called Reid a "dirty liar" for leveling the accusations against Romney, the presumptive Republican presidential nominee.

"I just cannot believe that the majority leader of the United States would take the floor twice, make accusations that are absolutely unfounded in my view and, quite frankly, making things up to divert the campaign away from the real issues," Graham said on CNN's "State of the Union."

Republicans and Democrats spent much of the Sunday talk shows sparring over Reid's claims about Romney's taxes -- accusations the former Massachusetts governor has denied. President Obama's advisers and supporters have again pressed Romney to release his tax returns in the wake of Reid's claims. Romney, who has released one year of personal income tax returns and has promised a second, has challenged Reid to "put up or shut up" and reveal his source.

Reid, D-Nev., made the allegations in a Senate floor speech last week. His office repeated the claim in an e-mailed statement on Sunday.

The e-mail, from Reid spokesman Adam Jentleson, said the information comes from "an extremely credible source" and called it "sad" that Republicans were being forced to defend Romney's decision not to release more tax returns.

Romney has released income tax returns from 2010, showing that he paid an effective tax rate of 13.9 percent on investment income that year. He has promised to release his 2011 returns when accountants are finished, and he has rejected calls by Democrats and some in his own party to disclose more.

Priebus, appearing on ABC's "This Week" said he wouldn't respond to a "dirty liar who hasn't filed a single page of tax returns himself."

Obama campaign strategist David Axelrod said on "Fox News Sunday" that he didn't "know who Harry was talking to." Romney "can resolve this in 10 seconds," Axelrod said. "Why don't they just put this to rest? What is it he's hiding?"

GOP Virginia Gov. Bob McDonnell said on CBS's "Face the Nation" that Reid had made a "reckless and slanderous charge."

"People don't care about Mitt Romney's tax returns," McDonnell said. "They care about their own tax returns."

The Washington Post contributed to this report.