BIRMINGHAM, Ala. - Rick Santorum lashed out at Mitt Romney as "sad," "desperate" and living in an "imaginary world" for suggesting he is too liberal to be Romney's running mate should the former Massachusetts governor become the Republican presidential nominee.

"I'm too liberal?" Santorum said in an interview with ABC News. "This is the imaginary world of Mitt Romney's ideology. It's just sad."

Calling Romney "one of the most liberal records ever to run in the post-Reagan era for president," Santorum said: "If I am too liberal to be Mitt Romney's running mate, oh my goodness. That just tells you how desperate he is."

Santorum was referring to an interview Romney did Monday with Neil Cavuto on Fox News. Cavuto asked Romney whether he would have to pick a running mate more conservative than he is to satisfy Republicans who think he is too moderate.

"That would preclude, of course, Rick Santorum," Romney told Cavuto. "Rick Santorum is not a person who is an economic conservative to my right."

Romney added: "I give him credit for being a conservative, but not a fiscal conservative. His record suggests that he does not have the fiscal conservative chops that I have."

Romney also told Cavuto that a drawn-out nomination battle that went all the way to the Republican convention in August would doom the party's chances of beating Barack Obama.

"If we go all the way to the convention, we would signal our doom in terms of replacing President Obama," Romney said.

It would take a miracle for Santorum to win the nomination, Romney said.

"If he is able to pull off a miracle, so be it, he'll be the nominee," Romney said. "You're going to see me getting the delegates I need to become the nominee and we sure as heck aren't going to go to the convention all the way to the end of August to select a nominee."

Asked about that, Santorum told ABC News: "You know, maybe he should start making the case for why he should be elected as opposed to playing all these math and convention games."