Yet the Clinton of lore, the once-in-a generation political natural who fought back to win his party’s nomination in 1992 and came through in clutch moments with great speeches over the years, has yet to appear.

In 1992, in a speech at a New Hampshire Elks lodge just before the state’s primary, he drove grown men to tears as he described their economic struggles. He memorably promised that if voters supported him, he would fight for them as president until “the last dog dies.”

Friends of Mr. Clinton’s who have seen him on the trail recently say that the ebullient energy he is known for — whether addressing a crowd or spending an hour on a rope line with voters — has matured into an elder statesman’s self-assurance. He may not electrify a room the way he once did, they say, but he is still an effective advocate for Mrs. Clinton.

“His age, his heart surgery, his veganism — I think it’s all brought a calmness into his life,” said George Bruno, a former Democratic Party chairman in New Hampshire and longtime ally of Mr. Clinton, referring to the former president’s quadruple bypass operation in 2004 and his strict plant-based diet. While some friends say Mr. Clinton would look zippier if he ate the occasional cheeseburger — an old favorite — others say he has never been healthier, with his weight down and his heart in excellent condition.

“He’s not as fiery as he once was, but he has an air of real self-confidence,” Mr. Bruno said.

Charismatic and soulful, and with an emotional force that could transfix voters, Mr. Clinton was among the most gifted of modern presidents at reading an audience and making points in a plain-spoken yet artful way. He thought of his speeches as “little talks,” he told friends, in which he could tell stories and share ideas in a conversational manner that people could relate to while never feeling condescended to. He was not a shouter, as Mrs. Clinton and Mr. Sanders can be. Rather, he would squeeze every word for maximum effect, mixing in humor and smiles, and train his eyes on audience members to make them feel that he was talking directly to them.

He could also shrewdly dismantle an opponent, but so far, Mr. Clinton does not seem to be relishing the fight. He has chosen to shrug off recent attacks from the leading Republican candidate, Donald J. Trump, and has been relatively mild in his critiques of Mr. Sanders.