But the terrain is tricky: Mr. Clinton is never as enraged or unpredictable as when his wife comes under attack. He will also remain focused on the Clinton Foundation, which has come under criticism for its fund-raising practices. (While Mrs. Clinton is likely to be campaigning in the next several months, Mr. Clinton will have a packed schedule of foundation-related events, including a big trip to Africa with supporters of his charity.) He runs the risk of reinforcing a Republican argument that the Clintons are America’s baby boomer past and do not represent a generational change in leadership. And he remains a wild card: What part would he play if he returned to the White House as first gentleman?

“I can’t think of anything tougher than being him in Secretary Clinton’s presidential campaign, because no matter what he does, people will take exception to it,” said Jerry Crawford, who was Mrs. Clinton’s Midwest co-chairman in 2008 and Mr. Clinton’s Iowa state director in 1992 and 1996.

“But at the same time, who would you rather have as a messenger and a strategist?” added Mr. Crawford, who speaks to the Clintons regularly. “And he learned a lot in 2008. First and foremost, he found out the last time how hard it is to be objective when it’s your spouse running.”

In the years since the bruising 2008 primary campaign, Mr. Clinton has regained his image as a Democratic stalwart, particularly by being a devoted surrogate to Mr. Obama and delivering a much-praised speech at the party’s convention in 2012. Fifty-six percent of Americans have a positive opinion of Mr. Clinton, making him more popular than both Mr. Obama and Mrs. Clinton, according to a Wall Street Journal-NBC News poll released this month. Mr. Clinton’s state of mind these days, according to people who are associated with him, can be summed up as Zen and jolly.

Mr. Clinton has also brushed up on how modern campaigns are waged. “The Victory Lab: The Secret Science of Winning Campaigns,” a 2012 book by Sasha Issenberg about how data and analytics have upended presidential campaigns, caught Mr. Clinton’s eye in an article he read. Still, Mr. Clinton often cannot help being himself. And nothing animates him more than offering his views on political strategy, sometimes with surprising company.

In September 2012, when Mitt Romney, the Republican presidential nominee, spoke at the annual Clinton Global Initiative gathering in New York, Mr. Clinton gave him advice backstage about how to appear in command when facing off against Mr. Obama in their coming debates.