“You haven’t given me the message,” Wallace said.

“Well, I don't have the — it would take me too long to go through all of that, but let me say it this way,” Ryan replied. “You can lower tax rates by 20 percent across the board by closing loopholes and still have preferences for the middle class for things like charitable deductions, for home purchases, for health care. So what we're saying is people are going to get lower tax rates and therefore, they will not send as much money to Washington and they'll keep it and decide for themselves."

Ryan gave a similar response on Tuesday.

“When you get in a math conversation it can take a little while,” Ryan said, before recounting the Romney campaign’s five-point economic plan centered around energy independence, education and job training, deficit reduction, addressing unfair trade practices, and encouraging small business growth.

“The problem is it took me 5 minutes with you and we’re on a 30 second TV show,” Ryan added.

The GOP vice presidential nominee also discussed specifics of the plan in an interview with Bloomberg News on Tuesday, saying the proposal will go after tax deductions and said clearly that individuals making $150,000 per year will see a net tax cut.

To accomplish this feat, Romney backs going after tax breaks “primarily” for upper-income individuals, Ryan said.

“You can lower tax rates across the board by 20 percent, primarily limit your deductions from those at the top end, and then get rid of all those nook and cranny special-interest tax expenditures ... and you can do this without raising taxes on middle-class taxpayers. This actually lowers their taxes,” he said.

The Obama campaign, which has continuously argued that the “math doesn’t add up” on the GOP plan, pounced on the Iowa event, saying in a statement that “Ryan can’t attend his own campaign rallies without being called out for failing to provide specifics about what Mitt Romney would do if elected.”

“That’s because just one day before the first debate, Mitt Romney has refused to say which deductions he’d cut for the middle class in order to pay for his $250,000 tax cuts for multi-millionaires,” spokesman Danny Kanner said in a statement. “And he’s refused to say how he’d replace ObamaCare or Wall Street reform to protect middle-class families or prevent the big banks from writing their own rules again. They won’t share those details with the country because they know that the details are bad for middle-class Americans. But tomorrow night, Mitt Romney will need more than 'zingers' and attacks on the president – he’ll need specifics – and his own supporters clearly agree.”

— Erik Wasson contributed

— This story was updated at 12:25 p.m.