WASHINGTON -- For the first time publicly, the Mitt Romney campaign was asked Sunday to defend the six studies it routinely cites as supportive of the candidate's tax plan.

The studies have been called into question for weeks now, as only one or two of them are actually academic. The rest are blog posts and op-eds, some written by the same author, others by conservative sources. One study cited was actually paid for by the campaign itself, though the campaign has since replaced that study with another.

More problematic for Romney is that a number of them reached conclusions that he would find uncomfortable. Harvard economist Martin Feldstein, for instance, said that Romney's tax plan could work mathematically if it eliminated deductions and exemptions for individuals making over $100,000 per year. A Princeton study put that figure at $200,000, though the author told Bloomberg News that the figure may need to be brought down to pay for Romney's 20 percent across-the-board reduction in tax rates.

Still, the Romney campaign continues to cite those studies, including during the presidential debates. On Sunday, Fox News' Chris Wallace asked top adviser Ed Gillespie whether that was misleading.

Gillespie: These are very credible sources, and, you know...

Wallace: One of them is from a guy who is – is a blog from a guy who was a top advisor to George W. Bush. So these are hardly nonpartisan studies.

Gillespie: Look, Chris I think if you look at Harvard and AEI [American Enterprise Institute] and other studies are very credible sources for economic analysis

Wallace: You wouldn't say that AEI is a conservative think tank?

Gillespie: I would say it is a right-leaning think tank. That doesn't make it not credible.

Wallace: It doesn't make it nonpartisan.

Gillespie: It does make it nonpartisan. It's not a partisan organization, I can tell you, there are many instances where there have been things AEI came out with and said, I didn't find it to be necessarily to be helpful to the Republican Party.

Wallace: Would you say Brookings Institution is nonpartisan?

Gillespie: I would say the Brookings Institution is left leaning and nonpartisan.

Also on HuffPost:

Loading Slideshow...
  • With A Little Help From My Friends (Joe Cocker)

    <strong><a href="http://www.huffingtonpost.com/2012/05/28/mitt-romney-will-not-repu_n_1551540.html" target="_hplink">(May 28, 2012) --</a></strong> Despite a resurgence of <a href="http://www.thedailybeast.com/articles/2012/05/25/mitt-romney-s-new-bff-donald-trump.html" target="_hplink">Donald Trump's birther claims</a>, Romney refused to repudiate the billionaire, who has been helping with his 2012 campaign efforts. "You know, I don't agree with all the people who support me ... I need to get 50.1 percent or more and I'm appreciative to have the help of a lot of good people," Romney said. (Photo by Ethan Miller/Getty Images)

  • Who Let The Dogs Out (Baha Men)

    <strong><a href="http://www.huffingtonpost.com/2012/04/16/mitt-romney-seamus_n_1429925.html" target="_hplink">(April 16, 2012) -- </a></strong> In an interview with ABC's Diane Sawyer, Romney discussed the political fallout over strapping his dog Seamus to a car roof. He admits that he probably would not do it again. (Handout)

  • It's The End Of The World As We Know It (R.E.M.)

    <strong><a href="http://www.huffingtonpost.com/2012/04/04/mitt-romney-medicare-president-obama_n_1403267.html" target="_hplink">WASHINGTON, D.C. (April 4, 2012) --</a></strong> Speaking before the Newspaper Association of America, Romney attacked Barack Obama on his health record, claiming the president "has taken a series of steps that end Medicare as we know it." (Photo: Jim Watson/AFP/Getty Images)

  • For The Love Of Money (The O'Jays)

    <a href="http://www.huffingtonpost.com/2012/03/14/mitt-romney-i-made-a-lot-of-money_n_1345516.html" target="_hplink"><strong>NEW YORK, N.Y. (March 14, 2012) -- </strong></a> Romney became testy on Fox News while discussing his appeal to lower-income voters. On the same day, Occupy Wall Street protesters staged a demonstration outside Mitt's Waldorf Astoria hotel fundraiser. (Photo: Emmanuel Dunand/AFP/Getty Images)

  • People Are Strange (The Doors)

    <strong><a href="http://www.huffingtonpost.com/2012/03/09/mitt-romney-south_n_1334478.html" target="_hplink">PASCAGOULA, Miss. (March 9, 2012) --</a></strong> While on the trail in Alabama and Mississippi, Romney got in touch with his Southern side, learning how to say "y'all" and liking his grits. With those new experiences in hand, he admitted that "strange things are happening to me." (Photo: AP/Evan Vucci)

  • Pink Cadillac (Bruce Springsteen)

    <a href="http://www.huffingtonpost.com/2012/02/24/mitt-romney-cadillac_n_1299740.html" target="_hplink"><strong>DETROIT, Mich. (Feb. 24, 2012) -- </strong></a> While speaking before the Detroit Economic Club at Ford Field, Romney listed not two, not three, but four American-made cars that he and his wife, Ann, owned. Among the vehicles: "a couple of Cadillacs." (Photo: Scott Olson/Getty Images)

  • It's The Hard-Knock Life (Annie & The Orphans)

    <strong><a href="http://www.huffingtonpost.com/2012/02/01/mitt-romney-very-poor_n_1246557.html" target="_hplink">TAMPA, Fla. (Feb. 1, 2012) --</a></strong> In an interview with CNN, Romney noted that he is "not concerned about the very poor," citing the social safety net for that segment of the populace. (Photo: Ethan Miller/Getty Images)

  • America The Beautiful

    <a href="http://www.huffingtonpost.com/2012/01/31/mitt-romney-america-the-beautiful-_n_1243908.html" target="_hplink"><strong>THE VILLAGES, Fla. (Jan. 31, 2012) --</strong></a> On the eve of Florida's primary, Romney led his supporters in a singing of the patriotic song. (Photo: Emmanuel Dunand/AFP/Getty Images)

  • Successful (Drake, Lil Wayne)

    <a href="http://www.huffingtonpost.com/2012/01/19/mitt-romney-tax-returns_n_1217708.html" target="_hplink"> <strong>CHARLESTON, S.C. (Jan. 19, 2012) -- </strong></a> During CNN's GOP debate, Romney refused to commit to disclosing his tax returns, offering no apologies for his success. (Photo: Emmanuel Dunand/AFP/Getty Images)

  • Bye Bye Bye ('N Sync)

    <a href="http://www.huffingtonpost.com/2012/01/09/mitt-romney-i-like-being-able-to-fire-people_n_1194115.html" target="_hplink"><strong>NASHUA, N.H. (Jan. 9, 2012) -- </strong></a> In a speech about insurance options, Romney tells audience members, "I like being able to fire people who provide services to me." (Photo:AP/Charles Dharapak)

  • Don't Know Why (Norah Jones)

    <strong><a href="http://www.huffingtonpost.com/2011/10/28/mitt-romney-embraces-climate-change-denial_n_1063905.html" target="_hplink">PITTSBURGH, Pa. (Oct. 27, 2011) --</a></strong> Back in June 2011, Romney said humans are somewhat tied to climate change. By October, he had reversed course, saying "We don't know what causes climate change." (Photo: Jim Watson/AFP/Getty Images)