He boasted that he was recently declared an enemy in the English-language magazine of the Islamic State. “They know who I am, and I know who they are,” he said.

At times, his unwillingness to emphasize the issues that worked for him in 2012 — opposition to abortion and same-sex marriage — and his decision to criticize his own party for being obsessed with cutting taxes for the rich have made him seem the boldest candidate in the race.

In 2012, Mr. Santorum embodied many conservatives’ dissatisfaction with Mr. Romney, who won the nomination after other conservatives proved to have fatal flaws.

Until days before the Iowa caucuses, Mr. Santorum was an underfinanced outsider, but his dogged campaigning in all 99 of the state’s counties, often before tiny crowds, paid off. His 34-vote margin of victory was not officially announced until two weeks after the caucuses, something Mr. Santorum resents to this day for costing him momentum in later contests.

This time around, many of his senior aides and supporters have defected to other candidates.

“I worked for Santorum last time,” Cody Hoefert, the co-chairman of the Republican Party of Iowa, said with a tone of befuddlement. “The guy wins more states than anyone but Romney, and he’s polling at 1 percent.”

One reason for Mr. Santorum’s struggle this year is that in a field shaping up to include a dozen or more Republican contenders, including sitting governors and senators, he has not held office since 2007 and looks to many voters like someone who has already had his shot.