(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.
– Follow the Ticker on Twitter: @PoliticalTicker
– Check out the CNN Electoral Map and Calculator and game out your own strategy for November.
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.
– Ryan gives House GOP a 'pep talk'
– Defending Romney, McCain cites 2008 campaign trail misspeak
– Romney to ramp up campaign schedule with bus tour
– Pawlenty takes post at Financial Services Roundtable
– Letterman: No hard feelings toward Romney
Even McCain pick Palin over Romney. There was a reason for that. Mitt could stop this easily. I doubt he broke any laws, but it's hard to wear that flag lapel pin while paying half the percentage that the average American does. Especially when you lobbied for the loopholes that you enjoy.
That kind of cooperation is sure to translate into votes!!
Obama demanded that the Clintons release all tax returns - personal and otherwise - and they complied. Mitt needs to do the same.
"Put up or shut up" sounds like something Romney should do.
Tax returns, birth certificates, it's all the same.
An easy fix. You want to be president, show the American people what percentage you have paid compared to the rest of us.
This tax return issue will not go away. Release the tax returns and silent the uproar now. OR do you have something to hide?
"You people don't need to know who my sources are. You're not getting anything more out of me."
– Harry Reid 2012
Sorry, Mr. McCain, this conservative, says you Republicans crossed the line when you let Rush Limbaugh & Sean Hannity become your defacto voice of the Republican Party!
Mr. Rommey show your tax returns!
Is he for real?! You can't prove a negative. Romney is the one who needs to put up!!
Notice that McCain said..."there was nothing there to disqualify him". He isn't saying anything about his taxes. The old spin and spin thing. McCain, you saw them, be honest with America and stop with these half rear end explanations.
Mr. Romney is clearly hiding something, otherwise: why wouldn't he release the returns ?... Geez our "elected" leaders are really something..
If Obama had to produce his birth certificate–an outlandish demand– the least Mitt can do is produce taxes returns, like other candidates do.
Hey Mitt if you've got nothing to hide then you put up or shut up. Your dad had no problem releasing his tax returns so what's the issue with you doing it?
Just keeps getting worse for GOP candidate. If America chooses him as our best option at president, I will be baffled and disgraced.
We should have looked into the Kennedy's wealth and tax records when he was running for president. How about all the sealed records regarding President Obama to keep everything transparent.
John McCain saw the tax returns...could he be Harry Reid's source?
Which other way could an aspiring POTUS demonstrate his patriotism than paying his taxes fully and being proud to show them. Show us the tax returns, Mr. Mitt. We are waiting!!
We don't need to see his tax returns. The fact that he doesn't want to show them is good enough for me. Mitt doesn't want to show how little, if any, taxes he has paid in the past. Let's give him the 1-3% of the vote he deserves come November and send the Republican party the message they deserve to hear – "Their time as a viable political party in this country is over".
Show the tax returns!!!
Obama will not even show us his college application...
Dude, just show us the darned taxes!
Usually a bad sign for them when they retreat to the conservative talk show circuit.
HE is telling REID to put up or shut up? Desperation setting in here. It looks like these allegations may be true.
nothing to hide, then lets see your past tax records! You people demanded to see a birth certificate of an American citizen now lets see something that really matters.