WASHINGTON -- Republican vice presidential nominee Paul Ryan promised again on Sunday that Mitt Romney’s tax plan would bring lower tax rates for all Americans while remaining revenue-neutral, although he didn’t fully explain how it would accomplish that.

The Obama campaign has blasted the Romney-Ryan ticket for not providing details on how it would give Americans such large tax breaks without growing the deficit. Ryan reiterated in an interview on "Fox News Sunday" that the plan would drop taxpayers' bills by 20 percent without costing a dime, due to closed tax loopholes, but he was short on specifics when pressed by host Chris Wallace.

“You haven’t given me the math,” Wallace said in one exchange.

“I don’t have the ... It would take me too long to go through all of the math,” Ryan responded.

“But let me say it this way,” he went on. “You can lower tax rates 20 percent across the board by closing loopholes and still have preferences for the middle class for things like charitable deductions, home purchases, for health care. What we’re saying is people are going to get lower tax rates and therefore they will not send as much money to Washington.”

Wallace played a clip of President Barack Obama mocking the Romney campaign’s lack of details on the loopholes while on the campaign trail. “No matter how many times they tell you they’re gonna start talking specifics really soon, they don’t do it,” Obama said. “And the reason is because the math doesn’t work.”

A report from the Tax Policy Center found that Romney’s tax plan would actually raise taxes on many middle-class Americans, although the Romney campaign has dismissed the report as partisan.

Also on HuffPost:

Loading Slideshow...
  • Where Have You Been (Rihanna)

    US Republican presidential candidate and former Massachusetts Governor Mitt Romney (L) shakes hands with his running mate Wisconsin Representative Paul Ryan in front of the campaign bus prior to each boarding their own campaign airplanes at General Mitchell Airport in Milwaukee, Wisconsin, August 12, 2012. (Photo credit should read SAUL LOEB/AFP/GettyImages)

  • Happy Together (The Turtles)

    WAUKESHA, WI - AUGUST 12: Republican presidential candidate and former Massachusetts Gov. Mitt Romney and vice presidential candidate and Wisconsin native Rep. Paul Ryan (R-WI) (L) greet supporters during a campaign event at the Waukesha Expo Center on August 12, 2012 in Waukesha, Wisconsin. (Photo by Darren Hauck/Getty Images)

  • Right By My Side (Nicki Minaj, Chris Brown)

    Republican presidential candidate, former Massachusetts Gov. Mitt Romney, left, listens to his vice presidential running mate, Rep. Paul Ryan R-Wis., as they campaign Sunday, Aug. 12, 2012, in High Point, N.C. (AP Photo/Mary Altaffer)

  • Only Wanna Be With You (Hootie & The Blowfish)

    WAUKESHA, WI - AUGUST 12: Republican presidential candidate and former Massachusetts Gov. Mitt Romney and vice presidential candidate and Wisconsin native Rep. Paul Ryan (R-WI) (L) hug after speaking at a campaign event at the Waukesha Expo Center on August 12, 2012 in Waukesha, Wisconsin. (Photo by Darren Hauck/Getty Images)

  • I Cry (Flo Rida)

    WAUKESHA, WI - AUGUST 12: Republican presidential candidate and former Massachusetts Governor Mitt Romney (R) looks on as his running mate Rep. Paul Ryan (R-WI) wipes away tears during a homecoming campaign rally at the Waukesha County Expo Center on August 12, 2012 in Waukesha, Wisconsin. (Photo by Justin Sullivan/Getty Images)

  • You're My Best Friend (Queen)

    Republican presidential candidate, former Massachusetts Gov. Mitt Romney, left and running mate Rep. Paul Ryan, R-Wis., talk in the campaign bus before an event at the Waukesha county expo center, Sunday, Aug. 12, 2012 in Waukesha, Wis. (AP Photo/Mary Altaffer)

  • Wouldn't It Be Nice (The Beach Boys)

    WAUKESHA, WI - AUGUST 12: Republican presidential candidate and former Massachusetts Gov. Mitt Romney and vice presidential candidate and Wisconsin native Rep. Paul Ryan (R-WI) (L) greet supporters during a campaign event at the Waukesha Expo Center on August 12, 2012 in Waukesha, Wisconsin. (Photo by Darren Hauck/Getty Images)

  • I'll Stand By You (The Pretenders)

    Republican presidential candidate, former Massachusetts Gov. Mitt Romney, right, and his newly announced vice presidential running mate, Rep. Paul Ryan, R-Wis., left, greet supporters, during a campaign rally in Manassas, Va., Saturday, Aug. 11, 2012. (AP Photo/Pablo Martinez Monsivais)

  • Two Tickets To Paradise (Eddie Money)

    Republican presidential candidate, former Massachusetts Gov. Mitt Romney, right, and vice presidential running mate Rep. Paul Ryan R-Wis. are joined by Ronmey's granddaughter Chloe while talking to reporters on the charter flight between Virginia and North Carolina, Saturday, Aug. 11, 2012. (AP Photo/Mary Altaffer)

  • We Found Love (Rihanna)

    WAUKESHA, WI - AUGUST 12: Republican vice presidential candidate and Wisconsin native Rep. Paul Ryan (R-WI) (L) wipes away tears as he and presidential candidate and former Massachusetts Gov. Mitt Romney greet supporters during a campaign event at the Waukesha Expo Center on August 12, 2012 in Waukesha, Wisconsin. (Photo by Darren Hauck/Getty Images)