In a raspy whisper, Bill Clinton said words many Democrats never thought they would hear, telling a huge Barack Obama crowd: "I have given my voice in the service of my president."

So Clinton has -- literally, with his exhaustive campaigning for a candidate he once disdained, and figuratively, by laying out a plain-spoken case for the current commander-in-chief, which Obama has sometimes struggled to match.

The ex-president, his white mane picked out by spotlights, was the warm-up act for Obama at a rally in swing state Virginia late Saturday that was packed despite frigid temperatures, just three days before America votes.

An hour later, as Clinton's theme song "Don't Stop" by Fleetwood Mac blasted into the night air, the business of placing his two-term Democratic legacy on Obama's shoulders was complete.

The joint appearances -- Clinton introduced Obama in Concord, New Hampshire on Sunday -- are the latest effort by the Obama campaign to catch a ride on Clinton's reputation for engineering nearly a decade of prosperity in the late 1990s.

They also highlighted the growing warmth between the two party giants, following tensions that flared when Obama thwarted Hillary Clinton's bid for the Democratic nomination in 2008.

Bill Clinton "has been a tireless -- and by tireless I mean he is also losing his voice -- advocate on behalf of the president," Obama campaign spokeswoman Jen Psaki told reporters Sunday aboard Air Force One, as the past and current presidents sat together in the front of the plane.

"Obama has enjoyed the growth in their friendship and relationship," Psaki added. "They have a really easy rapport with one another and that's been a really enjoyable part of the last couple of weeks."

Clinton, the only Democrat elected to two White House terms since World War II, made the case for Obama in his folksy southern way as he mockingly dismantled Republican nominee Mitt Romney's credentials.

The former president, of whom it was once said "he could talk a monkey out of a tree," reprised a role he played when he brought the house down at the Democratic National Convention in September.

He spoke of Obama's approach to economics, the middle class, and to being America's commander-in-chief, almost as an older man would of a protege.

"He has the right philosophy about how to fix the mess we're in," Clinton told a big crowd in Concord, referring to the steps the president has taken to bring America back from the brink of depression after the 2008 financial crisis.

And Obama's handling of the Hurricane Sandy crisis just days before the November 6 election? "It was a stunning example of how 'we're all in this together' is a way better philosophy than 'you're on your own,'" Clinton said.

He also took Romney to task for running an ad in Ohio that claimed Chrysler planned to outsource production of its Jeep vehicles to China, which prompted company bosses to lash Romney for pushing a falsehood.

"You know, when I was young, and I got my hand caught in the cookie jar, my face reddened, right, and I took my hand out the cookie jar," Clinton said.

"Governor Romney, he's just digging for more cookies."

Clinton has now done 28 events for Obama in the climax of the campaign, using his popularity to serve as a powerful advocate for the president.

When his turn came in Virginia, Obama showered warm personal praise on his Democratic predecessor as he bounded on stage after the ex-president's speech, and gave him a warm handshake and a hug.

"I was just sitting there soaking it all up, he was a great president, he has been a great friend," Obama said.

Clinton's embrace of Obama, after a cool relationship in the first years of the younger man's term, has revived one of the great imponderables of US politics -- what are the Clintons up to?

Some conspiracy theorists believe the former president and master campaigner is backing Obama so warmly simply to secure Obama's support should his wife Hillary decide to make another run for the presidency in 2016.

Others believe that Clinton, grateful for Obama's choice of his wife as secretary of state, simply could not resist the call to action, to the frontline campaign politics he loves.

Clinton also has much to gain politically. Many of the young people who Obama inspired to get involved in politics in 2008 were too young to have a clear memory of Clinton's administration.

Plunging into the crowd with Obama on Saturday, Clinton was introducing himself, and more importantly his political legacy, to a whole new generation.

Clinton must also be luxuriating in the sight of Obama, who was once sniffy about his 1993-2001 presidency, now gushing with praise for his record.

[Image via Agence France-Presse]