Former House Speaker Newt Gingrich told a news conference in Concord, N.H., Romney was a political chameleon liberal, willing to change positions to suit his needs.
"The idea that he's electable is just silly," Gingrich said.
"You can get all the Doles and all the McCains in the world, as Romney probably will," Huntsman said, referring to former Sen. Bob Dole of Kansas, the party's 1996 nominee, who also backed Romney. "But in the end, who cares?"
And Rick Santorum, who lost to Romney in Tuesday's Iowa caucus by just eight votes, told supporters in an e-mail Wednesday Romney was a "bland, boring career politician who will lose to Barack Obama."
Rep. Michele Bachmann of Minnesota announced Wednesday she would drop out of the race after her sixth-place Iowa finish.
Texas Gov. Rick Perry, who finished fifth in Iowa despite spending more than any other candidate, said he was still in the race and would travel to South Carolina to campaign ahead of that state's Jan. 21 primary.
Romney, McCain and South Carolina Gov. Nikki Haley were to campaign together in Charleston Thursday.
McCain is a former fighter pilot and prisoner of war who won military-heavy South Carolina in 2008.
McCain and Haley were to follow Romney to New Hampshire for appearances Friday and Saturday in advance of a Saturday New Hampshire GOP contenders' debate, Romney's campaign said.
"Do we think we can get more than an eight-vote margin here in New Hampshire?" Romney asked a crowd in Manchester Wednesday, a joking reference to his record slim Iowa victory margin.
Santorum capitalized on his Iowa success by raising $1 million Wednesday, Santorum campaign manager Mike Biundo said.
Rivals noted that even raising $1 million a day would still leave him far behind Romney, given the little amount of money Santorum had before.
Biundo told The New York Times the campaign was hiring a national coalition coordinator to help bring the conservative movement's disparate elements together.
Obama adviser David Axelrod also attacked Romney Wednesday.
"Taking two positions on every issue -- one on the left and one on the far right -- doesn't make you a centrist," Axelrod, the architect of Obama's 2008 victory, told reporters in a conference call Wednesday. "It makes you a charlatan."
Axelrod said it was wrong to think Romney was the presumptive Republican nominee, saying three-quarters of Iowa caucus voters chose someone other than Romney.
"He's still the 25 percent man," Axelrod said.