Clinton, as popular now as when he was inaugurated in 1993, retains a hold over US voters and was especially valuable in reaching working-class whites in key states like Ohio and Pennsylvania that were a tough sell for Obama.

"If you vote your hopes and not your fears, if you vote for unity and not division, if you think we can all work together, you will all re-elect Barack Obama president of the United States," the snowy-haired Clinton told a Monday night rally in Pittsburgh, Pennsylvania.

That message apparently carried the day on Tuesday, when voters re-elected Obama over Republican challenger Mitt Romney by substantial margins even in swing states that had appeared too close to call on election day.

Tuesday night, after receiving a call from Romney conceding defeat, Obama got on the phone and made his first call to Clinton, a campaign official said.

Their remarkable political reconciliation unites two once bitter rivals who as a team of temperamental opposites could give new energy and direction to Obama's second term, at a time when the US faces huge - and divisive - economic challenges.

If the partnership holds, it might also pave the way for another Clinton in 2016.

Hillary Clinton, who is expected to step down as secretary of state, has said she was not thinking about another run for the presidency, but her husband isn't so sure.

"I have no earthly idea what she will decide to do," Bill Clinton said in an interview with CBS in September. "I think we ought to give her a chance to organise her life and decide what she wants to do."

Clashing presidential ambitions were at the root of the Obama-Clinton rivalry, which pitted an upstart one-term Illinois senator against two towering Democratic figures.

The former president was almost contemptuously dismissive of Obama when he defeated Hillary in a fierce fight for the Democratic presidential nomination in 2008. Bill Clinton called Obama's victory a "fairy tale".

But as president, Obama invited Hillary onto his "team of rivals" as secretary of state, a generous gesture that at once held the power couple in check and left the door open to a more genuine reconciliation.

Still, it wasn't until this September, with the president under fire for a flagging economy and high unemployment, that Bill Clinton fully embraced Obama in a speech at the Democratic nominating convention, laying out with conviction why Americans should vote for him.

Even Obama, who turned in a lacklustre nominating speech himself, acknowledged that Clinton had made the case for re-election better than he had.

Obama said he should name Clinton to a new position known as secretary for explaining stuff.

And Clinton lit up the hustings for Obama, his white mane, animated face and folksy turns of phrases captivating undecided voters, many of whom could still remember the era of peace and economic prosperity during his 1993-2001 presidency.

"You have to admire the Clintons. Bill and Hillary Clinton have done more to re-elect Barack Obama than Barack Obama has," marvelled former House speaker Newt Gingrich on Fox News.

Bill Clinton, 66, jumped into a hopscotch of battleground states in what - depending on his wife's future plans - may or may not be his last campaign tour as a super surrogate.

He also included a fair amount in his speeches about Bill Clinton: his enthusiasm (higher than four years ago); his legacy ("I am the only living former president that ever gave you a budget surplus"); and, yes, his wife, the mention of whom brings big applause and the occasional "we love you, Hillary" from the crowd.

The 2012 campaign solidified (or restored) Clinton's status as the hardest working man in a game he loves and plays like no one else. "The master, Bill Clinton," Obama called him last week, hailing Clinton as "a great president and a great friend".

White House aides spoke of a bond between the two that grew during the final days of the campaign, when a raspy-voiced Clinton joined Obama in a last push.

He kept coughing, patting his chest and mouthing words that carried only muffled strains in chilly air. Black tea with honey and a steady diet of cough drops between events helped little.

"I have given my voice in the service of my president," Clinton told a huge Obama crowd in Concord, New Hampshire, as the campaign came to a close.

Out of public view, the former president was equally tireless. In addition to headlining 37 rallies for Obama over the last seven weeks of the campaign, Clinton was serving as a back-channel strategist for the re-election enterprise.

But subtext will inevitably abound in the Clinton orbit. For starters, while his "friendship" with the current president is clearly improved, it remains a source of intrigue given the strains that were sown during Obama's primary run against Hillary in 2008.

"This is not about relationships," said Terry McAuliffe, Bill Clinton's close friend and a possible candidate for governor of Virginia, where he accompanied Clinton last weekend.

"This is bigger than that."

By "bigger", McAuliffe meant that Clinton is chiefly concerned with the direction of the country, not his relationship with anyone.

It also calls to mind a maxim uttered often among Democrats, and not always with reverence: that it is "all about the Clintons".

And it is not difficult to view Bill Clinton's investment in Obama's re-election without an eye to whether Hillary Clinton runs in 2016.

It is also impossible to miss what is Bill Clinton's most reliable applause line on the stump, and something he manages to belt out in a full voice.

"By the way," he says in praising Obama's foreign policy record, "he's got a heck of a secretary of state, too."

And the crowd always goes wild.

If there has been one enduring lesson from his career, it is that the Big Dog is resilient. He can be disgraced, impeached, defeated - but he comes back.

The full spectacle of this was on riveting, if raspy, display in the closing days of the presidential campaign.