Former House speaker Newt Gingrich is doubling down on his criticism of Rep. Ron Paul, calling the Texas Republican “a protest candidate” who lacks the potential to win the presidency.

Former House Speaker of the House Newt Gingrich (R-Ga.) says Ron Paul won’t be the GOP presidential nominee. (Brendan Smialowski/Getty Images)

As polls show Gingrich’s presidential bid continues to decline while Paul’s gains traction in Iowa, the former House speaker took aim at his rival’s libertarian views on social and foreign policy issues, arguing that voters are “not going to elect somebody who believes in legalizing drugs.”

“]Fox News contributor] Dick Morris said last night on a show that he thought, on five different major issues, he was more liberal than Barack Obama,” Gingrich said. “And I think you have to look at all this. Ron Paul is a protest candidate. He’s a serious protest candidate. I think he sincerely believes what he says. But if you look at his total program, I think it is virtually impossible for him to be nominated by the Republican Party.”

Gingrich’s comments come as most of the GOP presidential field – with the notable exception of former Massachusetts governor Mitt Romney – is now throwing the kitchen sink at Paul, who polls suggest stands a good chance of winning or placing second in next week’s Iowa caucuses.

In an interview with Real Clear Politics on Thursday, Romney said that he will support whoever is the eventual GOP nominee – but he emphasized that in his view, “Ron Paul’s not going to be our nominee.”

Former Utah governor Jon Huntsman (R) struck a similar note, telling reporters that “if Ron Paul gets to the finish line I’d be happy to support him ... but he is not going to get there,” according to NBC News’ Jo Ling Kent.

As for his own bid, Gingrich on Thursday downplayed expectations for Iowa, where he has dropped from frontrunner to fourth place in the past three weeks.

“I think we’ll be right in the middle of the hunt, and I think we’ll come out of Iowa with enough strength to go to New Hampshire and we’ll come out of New Hampshire with enough strength to go to South Carolina,” Gingrich told CNBC’s Kudlow. “And I feel pretty confident that we’ll win South Carolina because I don’t think that a Massachusetts moderate is going to play very well in South Carolina.”