The two facets of Mr. Gingrich — the elder statesman who swears off negative campaigning versus the still sharp-tongued firebrand who enthralls many voters — often seem to be warring in full public view. He prefaced an answer at a debate in Iowa last week by saying he was “editing” his words so as not to come off as intemperate.

Fergus Cullen, a former chairman of the Republican Party of New Hampshire, compared Mr. Gingrich to a recovering alcoholic who has recently left rehab.

“Yes, he could be disciplined this week, but that doesn’t mean he won’t fall off the wagon or have a relapse next week,” Mr. Cullen said. “That’s the concern a lot of people who respect Gingrich have.”

Friends and close associates call that view a caricature, and they point to the many ways Mr. Gingrich’s life changed when he was forced from the speakership, resigning from the House in 1999 amid widespread perceptions of grandiosity and a tendency to self-destruct.

“He was in a very heady position as speaker, and then he lost power and that was a very humbling experience,” said Rick Tyler, a former aide who began working with Mr. Gingrich in that period. Leaving the House led to a period of soul-searching and ultimately Mr. Gingrich’s conversion to Roman Catholicism in 2009.

“He has definitely changed,” said Mr. Tyler, who this week joined an independent ”super PAC” to support the Gingrich campaign. “Newt is more grounded, more thoughtful and sees things less from ‘I’m going to change the world’ than seeing himself as subject to his creator.”

His conversion followed years of attending Mass to hear his wife sing in the choir of the Basilica of the National Shrine of the Immaculate Conception in Washington. He was a familiar figure reading in the cafeteria or a pew during the 90-minute choir rehearsals before the noon Mass.