Concord, N.H. ? With the hours slipping away before New Hampshire’s presidential primary, Republican rivals fought on multiple fronts Sunday to slow Mitt Romney’s march toward his party’s nomination.

Their efforts were on display in a combative morning debate and in campaign stops across the state amid the growing belief that the window to stop Romney’s momentum was closing. Having narrowly won last week’s Iowa caucuses, the former Massachusetts governor is the overwhelming front-runner in New Hampshire’s election Tuesday, the first presidential primary election in the nation, and is poised to do well in the subsequent contests.

“The case for the alternative is rapidly disappearing,” Romney adviser Tom Rath said.

With that fear in mind, the Republican contenders fanned out across the state Sunday to deliver their closing arguments directly to voters.

Newt Gingrich, the former House speaker, blasted Romney as a “Massachusetts moderate” and warned that a video being released by his allies would attack Romney’s business career.

“To quote the governor, you have to have broad shoulders, and you have to be able to take the heat to be in the kitchen,” Gingrich said.

If Gingrich was Romney’s chief critic, he was hardly alone.

Former Pennsylvania Sen. Rick Santorum pointedly asked Romney during the debate why he hadn’t sought re-election after one term as governor in the neighboring state.

Romney fired back with a reference to Santorum’s lucrative career in the six years since he lost his Senate seat. Describing politicians who lose office but stay in Washington “and make money as lobbyists or conducting their businesses,” Romney said, “I think it stinks.”

Romney won the Iowa caucuses last Tuesday by a scant eight votes over Santorum, but he is so far ahead in New Hampshire polls that his rivals have virtually conceded he will win. But they’ve also joined with an unlikely ally in fueling an evolving expectations game.

“If Mitt Romney doesn’t get over 50 percent on Tuesday here, being a former governor of the state right next door and having a family home here, then there’s something seriously wrong,” said Democratic National Committee Chairwoman Debbie Wasserman Schultz, who spoke to reporters outside the debate hall.

An increasingly confident Romney campaign countered by highlighting the possibility of back-to-back victories in Iowa and New Hampshire.

“If Mitt wins, I think the history-making nature of that win will overwhelm all the other coverage of the race to this point,” senior Romney adviser Eric Fehrnstrom said. “No non-incumbent Republican has won both Iowa and New Hampshire.”

New Hampshire success has traditionally helped shape the outcome of the subsequent contest in South Carolina, which holds the South’s first primary Jan. 21.