(CNN) - Mitt Romney on Thursday pushed back against Senate Majority Leader Harry Reid's allegations that the presumptive GOP nominee has not been paying his taxes.
"It's time for Harry to put up or shut up," Romney said on Sean Hannity's radio show.
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In a Huffington Post interview published Tuesday, the leading Democrat said that an unnamed investor with Bain Capital, Romney's former private equity firm, told Reid in a phone conversation that Romney had not paid taxes for a decade.
Reid stood by the allegations on Thursday while speaking on the Senate floor.
"The word's out that he hasn't paid any taxes for 10 years," he said. "Let him prove he has paid taxes because he hasn't."
Democrats and President Barack Obama's re-election campaign have repeatedly called for Romney to release more tax documents to answer any questions about his financial portfolio.
Romney has made public the full 2010 tax return and an estimate for 2011, with a pledge to release the full year once it's complete. He filed an extension for an October deadline.
Critics have hit back against Reid's comments this week, faulting him for making claims based off an anonymous source.
"Harry's gonna have to describe who it is he spoke with because of course that's totally and completely wrong," Romney said Thursday in the radio interview. "It's untrue, dishonest and inaccurate. It's wrong. So I'm looking forward to have Harry reveal his sources and we'll probably find out that it's the White House."
Mirroring Romney's comments, senior campaign adviser Eric Fehrnstrom described Reid's comments as "shameful, pathetic, baseless and untrue" and argued the majority leader, with the help of the Obama campaign, was attempting to "engage in McCarthy-like accusations about Mitt Romney."
"Four years ago Barack Obama said he was going to come to Washington and change the politics. Instead the Obama campaign has turned into a bottomless pit. Every day they reach a new low," he said Thursday on CNN's "The Situation Room."
Adding on, Fehrnstrom said Romney has "never zeroed out his tax liability" and "has always paid 100% of what was owed."
After Romney's interview on Hannity's radio show, the majority leader responded in a statement and said he is not backing down from his remarks.
"As I said before, I was told by an extremely credible source that Romney has not paid taxes for ten years. People who make as much money as Romney have many tricks at their disposal to avoid paying taxes," he said.
Reid argued Romney is "hiding something" and called on the presumptive GOP nominee to release more taxes.
"When it comes to answering the legitimate questions the American people have about whether he avoided paying his fair share in taxes or why he opened a Swiss bank account, Romney has shut up. But as a presidential candidate, it’s his obligation to put up and release several years’ worth of tax returns just like nominees of both parties have done for decades," Reid said.
According to a source familiar with Reid's thinking, the majority leader, a former boxer, considers himself a political street fighter now, willing to throw a punch and get hit back big time for a cause. In this case, the cause is making sure the topic of Romney's tax returns remains in the news, and in the campaign narrative.
The source said Reid is "happy to mix it up and put this back in the news" and added the "finger waggers of the Acela corridor won't dissuade him from speaking his mind."
Reid, the source said, is unconcerned about the hits he is taking on making unsubstantiated claims, because the Democratic base is fired up. The source says they know that anecdotally. It's not an accident that Reid said this to the Huffington Post, a friendly outlet, and did so as Romney was returning from his overseas trip.
"What happened when Romney returned? The first story he has to deal with is his tax returns," the source said.
One of Romney's biggest surrogates, Sen. John McCain of Arizona, came to Romney's defense on Thursday, saying Reid may have "gone over the line."
"I've known Sen. Reid for many, many years, and occasionally he displays some rather erratic behavior," McCain said on CNN's "Anderson Cooper 360." "To accuse someone of doing something without a shred of proof–that the allegation has any substance–is really something I, frankly, don't understand."
McCain, whose 2008 presidential campaign reviewed Romney's tax records as they considered him to be the vice presidential running mate, said last month he can "personally vouch" that there was "nothing disqualifying" in the tax returns of the presumptive GOP presidential nominee.
On Thursday, he urged Reid to come "forward" with his source.
"I think politics are tough. I enjoy the give and take but I think Harry might have gone over the line," McCain said.
Later, while visiting a fruit stand with his mother in Florida, Ryan responded to the negative reaction in the room.
"Entitlement reform has unfortunately been made very partisan by partisans, and so I have gotten that kind of reaction and unfortunately it's what we come to expect because the politics of reforming entitlements has become very bitter and it's very unfortunate," he told reporters in Lakeland.
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Amen... "put up or shut up, Harry Reid"!
Mitt show up your tax return or shut up.
The burden of proof lies with you, Willard the serial liar. What are you hiding??? Show us your tax returns going back to 2000 (especially 2009), and while you're at it, we need to see your long-form birth certificate too.
No Romney, you put up or go away.
Mitt, Release your tax returns and shut him up. Easy as 1...2...3.
Harry Reid should be investigated by the IRS Disclosing private information about anyones taxes to the public is an illegal act. He needs to dosclose his supposed source to the IRS or face charge himself.
Turn them over when we see Pesident Obama's College Transcripts. Only then.
Harry Reid is making accusations based on an anonymous source (probably from inside his own head),and declaring to the World that Romney needs to answer them. Well, Reid needs to either declare the source of illegal information from the IRS or bite his tongue. I will go so far, as should everyone, to say:"Put Up or Shut Up Harry'! This is just dirty politics (what else can you expect from him) and this from the party of civility, tolerance and clean(?) policy heralded by Obama's promises in 2008. Beware Harry, we know what is in your closet.
I'm sending money to Romney right now.
be careful what you wish for Mitt.
How about Harry Reid put out one of his tax returns and explain how he acquired his wealth?
Actually, it's time for Romney to put up or shut up!
Isn't it a felony to disclose someone's tax returns. I think the IRS needs to investigate.
Please Gov. Romney pay your tax before you ask for votes. Taxes first, votes second
What's Romney hiding???. Reagan published his 12 years of taxes without this fuss.
Mean while bain donates to the Obama campaign....
Earth to Romney: put up or shut up. It's untrue that you didn't pay any taxes? Prove it.
For one...its none of my business to see his tax returns...or Obamas..Unless there is a legal case...
Two...I agree...Reid put up or please for once just shut up
Where are harry's returns? Or Obamas college records? What are they hiding?
All politicans dodge paying taxes because of the tax loop holes they have created for themselves in the tax code.
Not a big fan of Senator Reid but c'mon, the person who needs to put or shut up is Romney.
Where are the tax returns already?
So Mitt show the tax returns and prove Reid is wrong....that seems like a pretty easy solution unless you are hiding something!
Mitt, You have a golden opportunity to make Harry Reid, the Democratic party and those calling for the release of your tax returns to look like fools. Just think what an advantage that would provide you in the race for the presidency. All you have to do is release your returns.
Or will releasing them create even greater problems for you?
I'm not convinced of Reid's "source" but it doesn't seem right that a candidate running for President will only release one year of completed tax returns. If I want to borrow money from a local bank for any reason, five years of tax returns would be a minimum requirement.
No, Mittens, YOU put up, or shut up. Harry Reid ain't the one hiding your tax returns, but then again, you wouldn't know, would you? Can you go check? There's a flight on Air Jamaica leaving in fifteen minutes.