Romney calls Gingrich 'extraordinarily unreliable'

(CNN) - One day after Mitt Romney blasted Newt Gingrich for showing "erratic outspokenness," the former Massachusetts governor again leveled another verbal affront on Tuesday, this time calling his opponent an "extraordinarily unreliable" conservative.

Romney's comments follow a string of attacks exchanged between him and the former House speaker, a battle that took center stage at a Republican presidential debate on Saturday and continued during the following two days.

But in an interview with the Washington Post on Tuesday, Romney sharpened his attack strategy on Gingrich once again.

"He has been an extraordinarily unreliable leader in the conservative world–not 16 or 17 years ago but in the last two to three years," Romney said. "And even during the campaign, the number of times he has moved from one spot to another has been remarkable. I think he's shown a level of unreliability as a conservative leader today."

Those words mirror a similar tune he took on Monday when he called out Gingrich for saying the Palestinians were an "invented people."

In an interview with the Fox News Channel, Romney said Gingrich displayed "erratic outspokenness, which may be great in a campaign but is not great for someone who's running for president of the United States, representing this country."

The two continued the back-and-forth on Monday, with both candidates hitting each others' background in the private sector.

Also in the Washington Post interview on Tuesday, Romney took a swing at Gingrich for doing a 2008 commercial on climate change with then-Democratic House Speaker Nancy Pelosi, a move Gingrich has since called one of his "dumbest" decisions in recent years.

Romney also disagreed with Gingrich over comments he made about House Budget Committee Chairman Paul Ryan's popular plan among Republicans to reform Medicare. Earlier this year, Gingrich described it as a "radical change" but quickly apologized to Ryan after the comment drew fire in conservative circles.

And while Romney seems to be mounting a plan to cast shadows over Gingrich's record, Gingrich on Tuesday called for the candidates and their supporters to resist negative campaigning.

He made a similar rallying cry on Saturday before the debate, only to break it hours later when he sparred with Romney on stage. Gingrich launched one of toughest lines of the night by saying Romney would be a "lifelong politician" if he hadn't lost his senate bid against Ted Kennedy in 1994.