"Since we don’t know how he got here, I don’t know how he can be stopped"

Representative Charles Bass, a New Hampshire Republican who was elected in the Gingrich-led wave of 1994, agreed. He recently endorsed Mr. Romney.

“I don’t see Speaker Gingrich having appeal with the independent vote, which is the largest vote in my state and it’s obviously my base along with the Republican vote,” Mr. Bass said — suggesting his own vulnerability if Mr. Gingrich heads the ticket.

“There’s nothing about Speaker Gingrich that looks any more disciplined than he was in the 1990s,” said Terry Nelson, formerly a top campaign strategist for President George W. Bush and Senator John McCain who is neutral after having worked for Tim Pawlenty, the former Minnesota governor who quit the race in August…

Stuart Rothenberg, an independent analyst of Congressional races, said that Mr. Gingrich could hurt other Republicans running next year. “There are some Republican insiders I talk to who think it would be a full-fledged blowout,” he said.