CHARLOTTE, N.C. — When Bill Clinton takes his star turn Wednesday night at the Democratic convention, it will be another step on a remarkable climb back to the pinnacle of American politics.

He will be opening a new chapter in a fraught relationship with President Barack Obama — one that was strained four years ago, has since been mended and could well influence the outcome of the November election.

And if everything goes the former president’s way, it could conceivably lead to another Clinton winning the White House in 2016. Secretary of State Hillary Clinton is not on the premises, in keeping with the diplomatic tradition of steering clear of partisan politics, but her husband’s ubiquitousness here would certainly come in handy during any future presidential try by her.

All this is possible because, nearly 12 years after leaving office still marred by impeachment, the former president is arguably the most popular figure on the political scene. His personal approval ratings have never been higher, easily exceeding Obama’s. His easy drawl is bombarding the airwaves in battleground state television ads broadcast by the Obama team.

Obama has asked Clinton to place his name in nomination, which makes him the first ex-president to have that honor and provides further proof, if any were needed, of his importance to the re-election effort.

Big fundraiser

Clinton is already raising money for Obama from wealthy donors and volunteering strategic advice. “He calls me frequently,” said a senior Obama campaign official in Chicago. “He is all the way in.”

He is also keeping the family business alive while his wife finishes her term as secretary of state. He has been making endorsements in down-ballot races and raising money for Democrats who backed her presidential campaign and could be in a position to help her again.

Secretary Clinton, one of the only figures on the national scene today whose aura rivals her husband’s, has seen her personal ratings rebound to near-record highs during her tenure as the nation’s senior diplomat. She has announced plans to return to private life after the 2012 election, prompting intense speculation about another bid for the Democratic nomination.

“Why wouldn’t she run?” House Democratic leader Nancy Pelosi has said, echoing the assessment of many others inside and outside Clinton circles. She would turn 69 in 2016, but even those who say she hasn’t made up her mind don’t think age would be an impediment.

Because she will be on the opposite side of the planet tonight — meeting with China’s leaders as part of a 10-day, six-nation trip — her husband will not only be promoting Obama and burnishing his own legacy in Charlotte. He’ll be her stand-in too, said Ann Lewis, a top Clinton White House aide and senior adviser in Secretary Clinton’s 2008 campaign. “He’s been practicing the role of spouse for several years,” she said. “He’s pretty good at it.”

Seventh convention speech

The former president once lamented that he never confronted a crisis serious enough to establish his greatness as president. But the worst economy since the Great Depression has cast a rosier view on his eight years in office. Borrowing his popularity, Obama has picked up speech lines from Clinton, who retains his gift for explaining complex concepts in simple terms.

“Nobody has a better grasp and understanding of the issues than this man,” Obama said at a New York fundraiser in June, with Clinton at his side.

Clinton’s address tonight will be his seventh in a row to a Democratic convention. The first, also a nominating speech, was in 1988. That night, he bombed. His biggest applause line: “In closing.” By then, two of the three major TV networks had already cut him off.

Today, at 66, he’s a slim slice of his once-beefy self — the product of exercise and a vegan diet after heart surgery. He’s busy with travel, philanthropic works, lucrative speaking gigs (more than $13 million in fees last year, a personal best) and undiminished lust for the political game.

His prominence in Obama’s campaign is partly a role reversal for the titans of the Democratic Party. In 2008, lingering tensions between the Clinton and Obama camps led many Democrats to wonder whether the former president would deliver a full-throated endorsement at the convention in Denver. (He did.) Back then, Obama was riding high, and Clinton’s most prominent campaign efforts didn’t come until the final days before the election.

On board early

This year, he was brought on board early, reflecting the hard reality of a re-election fight that will be more difficult than the one that brought Obama to power. Obama aides describe Clinton as a validator: someone who can reel in wavering Democrats (including conservative-leaning Clinton Democrats) and reach out to undecided voters. Obama often drops Clinton’s name into his speeches, arguing that his economic plan is just like the one that gave the country prosperity during the Clinton years.

Clinton has the potential to help with “important voting groups that Obama is struggling with,” including “whites, men, seniors and political independents,” according to a recent analysis of poll data by Lydia Saad of the Gallup organization.