For a while, it had seemed that Rick Santorum’s crabby days were behind him. Gone were the sarcastic potshots at reporters and peevish outbursts aimed at his political opponents. He had transformed into the Mr. Rogers of the presidential race: good-natured, self-deprecating and downright likable.

But that nice-guy image has gone down the drain lately, with a series of provocative remarks and testy exchanges that have coincided with his slipping presidential fortunes. He may have hit a low point Sunday, when he uttered an expletive in response to a question from a New York Times reporter.

Asked what he meant when he said in a speech that rival Mitt Romney was the “worst Republican in the country” to go up against President Obama, Santorum lashed back at reporter Jeff Zeleny in an exchange that was captured by CBS.

“Stop lying!” he responded. “I said he was the worst Republican to run on the issue of Obamacare. And that’s what I was talking about!” In case there was any doubt that he meant it, he suggested that if he saw such a statement in print, it would amount to “bull----.”

For a candidate seeking to be the conservative standard-bearer in the election, it is perhaps not the worst thing to be caught on tape cursing out a reporter from a mainstream newspaper. Santorum’s campaign attempted Monday to capi­tal­ize on the incident, sending out a fundraising letter accusing the Times of being a liberal bully.

But for people who have followed Santorum’s decades-long career in politics, it was the latest sign that Santorum is reverting back to a prickly persona that predated his rise in the polls.

“Most of us who followed his career were just stunned at how, for seven or eight months, he was remarkably disciplined,” said G. Terry Madonna, director of the Center for Politics and Public Affairs at Franklin and Marshall College in Lancaster, Pa. Lately, he said, he has seen signs of a more familiar Santorum. “He’s visceral, emotional, provocative,” Madonna said. “It’s who he is.”

Santorum, who spent 12 years in Congress, developed a reputation as a passionate but sometimes hotheaded advocate for his causes. He was known for scolding fellow Republicans for crossing the George W. Bush White House and for making sometimes inflammatory comments — such as when he was quoted criticizing AmeriCorps as a program “for hippie kids to stand around a campfire to hold hands and sing ‘Kumbaya’ at taxpayers’ expense.’’ (He later became a supporter of the program.)

There were hints of that ornery personality in the early days of the presidential race, when media coverage of his Iowa stumping was scant. At that time, Santorum often complained loudly during debates that he wasn’t getting enough questions and used his stump speeches to criticize the media for its lack of attention to his campaign.

Santorum’s cutting rhetoric and willingness to engage on virtually any subject has been part of his charm this year, helping to fuel his rise in the presidential race. He managed to couple it with a self-deprecating, sweater-vested persona that stood out in contrast with a professorial Newt Gingrich, an agitated Ron Paul and a formal Romney.

But as media scrutiny intensified and his primary defeats mounted, some of that good-natured edge has fallen away. Last month, he was criticized by some fellow Republicans for calling Obama a “snob” for wanting children to attend college, and he was forced to apologize for saying John F. Kennedy’s comments about separating church and state made him want to “throw up.”

Santorum has acknowledged his occasional hotheadedness, as he did at the conclusion of a particularly passionate debate performance during his failed 2006 Senate campaign reelection effort.

“You can see from this debate I’m a passionate guy. I’m tough, I’m a fighter,” he said in his closing statement. “But you know what? I’m an Italian kid from a steel town, what do you expect from me?”

Still, Santorum’s use of a curse word at the rope line at a Wisconsin event on Sunday suggested to some that he may be losing his cool as Romney appears increasingly likely to wrap up the nomination.

“What we’re seeing are signs that the nomination battle is effectively over,” said John Weaver, who was a senior adviser to the presidential bid of former Utah governor Jon Huntsman. “The lashing out at the media and grasping of straws . . . are indicators we’ve entered the walking dead period.”

In a CNN interview Monday, Santorum defended his language in his run-in with the Times, saying he had been asked the same question by several other reporters who he believed had been put up to it by the Romney campaign.

And he joked, “Any good conservative who hasn’t had a flare-up with the New York Times isn’t worth their salt.”

Staff writer Chris Cillizza contributed to this report.