Republican presidential candidates (L-R) former U.S. Senator Rick Santorum (R-PA) listens as former Massachusetts Governor Mitt Romney speaks during the Republican presidential candidates debate in Concord, New Hampshire, January 8, 2012.
Rick Santorum may be leading his rival Mitt Romney in the Michigan primary, but the former Massachusetts governor is gaining on his current chief rival for the GOP presidential nomination. REUTERS/Shannon Stapleton

Republican presidential candidates stepped up their attacks on rival Mitt Romney in a televised debate on Sunday morning -- a mere two days before primary-election voters in New Hampshire head to the polls -- and the front-runner mostly weathered the attacks.

Criticism of the former Massachusetts governor zeroed in on the perception that he would be the strongest candidate against Democratic President Barack Obama in November's election.

Wasting no time, former U.S. House of Representatives Speaker Newt Gingrich said Romney would have a very hard time getting elected.

There's a huge difference between a Reagan conservative and somebody who comes out of the Massachusetts culture who essentially has a moderate record, Gingrich said in the opening moments of the NBC/Facebook debate in Concord, N.H.

Opinion polls show Romney holds a wide lead in New Hampshire, which holds its first-in-the-nation primary election on Tuesday, and that he also leads in South Carolina, the next state in the nominating process.

If his record was so great as governor of Massachusetts, why didn't he run for re-election, Rick Santorum, a former U.S. senator from Pennsylvania, said of Romney, who launched his unsuccessful 2008 White House bid just weeks after leaving the statehouse after one term.

Santorum has been riding a wave of popularity after a narrow second-place finish behind Romney in the first Republican presidential nominating contest in Iowa last week.

Romney defended himself as a solid conservative who was in politics as a detour from his business career as a venture capitalist, and he kept his focus more on Obama than on his Republican rivals: I happen to believe that if we want to replace a lifetime politician like Barack Obama ... we've got to choose someone who is not a lifetime politician, who has not spent his entire career in Washington.

One of the biggest applause lines of the morning came from Jon Huntsman, who responded, albeit belatedly, to a comment that Romney had made about him in Saturday night's Republican debate in Manchester, N.H.

Romney had slapped Huntsman for implementing Obama's agenda as U.S. ambassador to China.

Addressing debate moderator David Gregory, Huntsman said: This country is divided, David, because of attitudes like that ... The American people are tired of partisan divisions.

The candidates, who are essentially jockeying for a strong second-place finish in New Hampshire behind Romney to gain some momentum heading to the South Carolina primary on Jan. 21, also attempted to take aim at each other.

When Gregory pointed out that U.S. Rep. Ron Paul of Texas has a thin legislative record despite a long career in Congress, Santorum found an opening.

He's never really passed anything of importance. He's never been able to accomplish anything. He has no track record. He's been out there on the margins, Santorum said.

(Additional reporting by Sam Youngman; Editing by Eric Beech)