Republican strategist Mary Matalin on Sunday attempted to lecture Nobel prize-winning economist Paul Krugman, saying he had "lied" by claiming Republican vice presidential nominee Paul Ryan's wanted to turn Medicare into a voucher system.

During panel discussion on ABC's This Week, Wall Street Journal columnist Peggy Noonan asserted that Republican presidential nominee Mitt Romney had held himself during last week's debate by appearing to be "a completely moderate, centrist figure."

"Except that everything he used to claim his centrism wasn't true," Krugman pointed out. "So, this is a question. Does that start to take its toll over the next few months... When you say my plan covers pre-existing conditions when it doesn't and when your own campaign has admitted in the past that it doesn't, what do you say? That's amazing."

"You have mischaracterized and you have lied about every position and every particular of the Ryan plan on Medicare," Matalin interrupted, "from the efficiency of the Medicare administration to calling it a voucher plan."

"It is a voucher plan," Krugman replied.

"You are hardly credible on calling somebody else a liar," Matalin quipped.

Krugman quickly returned to Romney's claim during the debate, that his health care plan covered pre-existing conditions.

"I just think that pre-existing thing was a defining moment," he observed. "It was saying this guy believes -- not only did he say something that isn't true, but something that his own campaign has admitted isn't true. And he can say it in front of 70 million people. That's amazing."

As Think Progress noted, Ryan himself has described his plan as "converting Medicare into defined contribution sort of voucher system."

Watch this video from ABC's This Week, broadcast Oct. 7, 2012.