The second debate in 12 hours for the six GOP presidential hopefuls was book-ended by moments of scrutiny for Mitt Romney, the former Massachusetts governor and prohibitive favorite to win New Hampshire's Tuesday primary.

Mitt Romney found himself under fire from conservative detractors in the opening and closing moments of an NBC News-Facebook debate, broadcast on "Meet the Press" Sunday morning. Former House Speaker Newt Gingrich and former Pennsylvania Sen. Rick Santorum led the charge against Romney, questioning just how conservative of a nominee he would be for Republicans.

The heat on Romney fizzled during the middle of the portion of the debate before re-emerging toward the end, when Romney and Gingrich did public battle over the negative ads run by various super PACs in Iowa and New Hampshire, which have affected the trajectory of the GOP campaign.

The scrutiny represented a last effort by the other five Republican presidential candidates to draw contrasts with Romney with just 48 hours to go until Tuesday's New Hampshire primary.

"If his record was so great as governor of Massachusetts, why didn't he run for re-election?" asked Santorum, who battled Romney to a virtual draw at last Tuesday's Iowa caucus. "If it was that great, why did you bail out?"

But Romney kept the focus on his own record and eschewed attacking candidates, especially Santorum and Gingrich, who are expected to pose little threat to his strong lead in advance of the primary.

"I'm very proud of my record and I think the one thing you can't fool the people of New Hampshire about is the record of a governor next door," Romney said in response to the pile-on, largely avoiding making direct attacks against his detractors.

At one point, though, when Santorum interrupted him, Romney snipped: "Rick, it's still my time."