Bill Clinton and Barack Obama have not always seen eye-to-eye. Here are seven defining moments in their relationship. (The Washington Post)

In one of his first acts as President Obama’s new health-care adviser, Chris Jennings traveled to Harlem last month to pay a visit to his old boss, Bill Clinton.

Armed with a PowerPoint presentation detailing how the new health-care law will go into effect, Jennings made his pitch: Obama needs your help, both to persuade millions of uninsured Americans to sign up for coverage and to combat Republican attempts to undermine the law.

Clinton tried and failed to enact an expansion of health-care coverage as president. Now Obama, who signed the Affordable Care Act into law, is relying on Clinton to help make it a legacy.

Here in Manhattan on Tuesday, Clinton will welcome Obama on stage at his annual celebrity-sprinkled charitable gathering. The two presidents, in a conversation led by Clinton, will discuss the merits of the health-care law, which has come to be known as Obamacare. The event comes three weeks after Clinton gave a 50-minute speech in his home state of Arkansas explaining how the system works and arguing that it makes the country stronger.

For Obama and Clinton, whose relationship has been tense and sometimes hostile, selling health care gives them a chance to nurse old wounds while also helping each other politically. Whether this fall’s high-stakes rollout of Obamacare is successful could define both Obama’s legacy and the contours of the 2016 presidential contest, which Clinton’s wife, Hillary Rodham Clinton, is considering entering.

“Bill Clinton is to politics like a 12-year-old boy is to baseball: There’s nothing he wouldn’t do to be in the game,” said Don Fowler, a longtime Clinton friend and former Democratic National Committee chairman. “That sense of being permitted to play and being appreciated and having people pay attention to him — that’s his ultimate motivation. That’s what this circumstance gives him.”

Tensions from 2008 ease

Fowler remembers how unlikely a Clinton-Obama partnership seemed five years ago, when Obama started besting Hillary Clinton in the Democratic primaries. Fowler was at Bill Clinton’s side in South Carolina in 2008 as he inflamed racial tensions by attacking Obama on the stump.

But, Fowler said, “Obama won, and [Clinton] understands better than anybody that you shouldn’t let one unhappy experience dictate a whole life relationship.”

The tensions between Clinton and Obama slowly eased. In 2009, Obama named Hillary Clinton his secretary of state. And by 2012, Bill Clinton had become one of the president’s most important and effective campaign surrogates, delivering a speech at the Democratic National Convention that some Democrats believe secured Obama’s reelection.

Now, Clinton is back in the game with health care.

“This is an issue that is very personal for him,” said Skip Rutherford, a Clinton friend and dean of the University of Arkansas Clinton School of Public Service. “Presidents have tried to make this issue work for decades, and I don’t think anyone tried any harder than Bill Clinton. He did everything he could to move this ball forward.”

When Clinton and Obama appear together at the Clinton Global Initiative on Tuesday — almost 20 years to the day after Clinton kicked off his own push for health-care reform in 1993 — Hillary Clinton is expected to introduce them.

As first lady, Clinton was the face of her husband’s unsuccessful legislative effort. And she has a lot on the line politically with the Obamacare rollout. If she runs in 2016, she will want Obamacare to be considered a success by voters of all stripes, not as the polarizing issue it is now.

In addition, the closer Obama gets to her husband, the better Clinton is positioned to be the heir apparent, said Hank Sheinkopf, a Democratic strategist in New York who once worked for Bill Clinton.

“Obama’s not endorsing Hillary Clinton, but he’s Clintonizing the Obama presidency on an issue that really matters,” Sheinkopf said. “If I were [Maryland Gov.] Martin O’Malley or Joe Biden, I’d be jumping through the ceiling right now. I’d be very angry with President Obama for being with the Clintons.”

Explainer in chief

The August trip to Bill Clinton’s office was attended by Jennings and a cadre of other senior White House aides, including communications director Jennifer Palmieri, political adviser David Simas and policy adviser Jeanne Lambrew.

The Obama aides briefed Clinton on their enrollment strategy and how they planned to reach different populations as insurance marketplaces open for business on Oct. 1. Palmieri said the former president was enthusiastic and engaged, asking a lot of questions.

“We asked President Clinton if he would get involved in health care as ‘secretary of explaining stuff,’ ” Palmieri said, using the moniker Clinton received after his DNC speech. “He’s particularly skilled at taking a complicated problem or issue and breaking it down for people in a relatable way.”

Clinton spent additional hours studying the law and enrollment details with his own advisers, including his chief of staff, Tina Flournoy, and his former White House chief of staff, John D. Podesta, who also informally advises the Obama White House. Jennings also accompanied Clinton to Little Rock for the Sept. 4 health-care speech.

“I think Clinton really believes that the public needs a big dose of education on this, and he’s happy to participate in it,” Podesta said. “He’s felt that people have been bombarded with a lot of misinformation and if you explain what this is all about, people will have a better appreciation and come to want to see it enacted.”

Six in 10 Americans do not understand what changes will occur as the new law takes effect, according to a Washington Post-ABC News poll this month. While 42 percent said they support the law, 52 percent said they oppose it and just 34 percent approve of the way the Obama administration is handling the implementation.

Divisions on health care long have split along party lines, but Clinton is one of the few political figures with crossover appeal. Nationally, over seven in 10 registered voters held a favorable view of Clinton in an April Fox News poll. Clinton was nearly universally liked among fellow Democrats, while nearly half of Republicans saw him positively. As Sheinkopf put it, “There is a romantic view of Clinton.”

Paul Begala, a former Clinton campaign strategist, said, “It’s a wonderful thing to be able to have someone who clearly knows the subject, who strongly supports it and who’s not a polarizing figure stand up and say, ‘I’ve looked at this carefully, and this is a good deal.’ ”

Scott Clement in Washington contributed to this report.