His high-profile role also gives him the chance to enhance his legacy as Democratic elder statesman and global humanitarian. He can build up political IOUs should his wife, Secretary of State Hillary Rodham Clinton, decide to run again for president down the road.
Out of office since 2001, Clinton is proving that he retains a strong appeal with voters, especially in conservative states where Democratic candidates aren't eager to appear with Obama. The ex-president is a leading expert in the art of the political comeback – a skill the struggling Obama could use now.
Also, there's this uncomfortable truth: Obama needs Clinton to generate support with white, working-class and independent voters who were drawn to Clinton and Republican presidential candidate Mitt Romney, but who haven't warmed to Obama.
"If there's one thing we've learned in this election season, by the way, it is that a few words from Bill Clinton can do a man a lot of good," Romney joked in remarks at the Clinton Global Initiative last month, a nod to Clinton's convention speech.
In Indianapolis on Friday to boost Senate Democratic hopeful Joe Donnelly, Clinton said he hadn't expected to be so involved in the 2012 campaign. But Hillary Clinton is busy as America's chief diplomat and daughter Chelsea works for a broadcast network, he noted.
"So you're stuck with me," Clinton told the crowd.
Obama himself has cracked that he should appoint Clinton "secretary of explaining stuff."
The lighthearted tone and determined cooperation between Obama and Clinton is hard-won and may not end with the 2012 campaign.
The bruising 2008 Democratic primary between Obama and Hillary Clinton damaged relations between the families. Obama's appointment of Hillary Clinton as secretary of state, and her successes in the job, healed the relationship enough for Clinton to step in to help with Obama's re-election.
If Obama has Bill Clinton to thank for a second term, one way to pay back the debt could be Obama's support for a Hillary Clinton campaign of the future.
Among voters generally, Bill Clinton is more appealing than Obama. A CBS News/New York Times poll in September found 66 percent of registered voters with a favorable view of Clinton, compared with 45 percent for Obama.
"He's got a way of communicating complex ideas in plain ways that no one else has," said Greg Haas, who ran Clinton's 1992 campaign in Ohio and who is Democratic chairman in Franklin County, which includes Columbus. "No one in my lifetime. He's in a class by himself."
Clinton symbolizes good economic times for many voters, including a federal budget that was balanced under his leadership. His convention speech rebutted Romney's attacks on Obama and accused Romney of pushing an economic plan that is doomed to fail. Clinton lends Obama credibility on the issue that voters consistently name as their top concern: the economy.
Clinton rocketed out of the party's convention in Charlotte, N.C., last month energized by the massive crowd's roaring reaction to his partly ad-libbed call to arms. Clinton plunged into a two-day swing in Florida and has since added stops for Obama in a string of political battlegrounds.
During one recent week, Clinton: joined Obama at a meeting with about a dozen supporters at a private residence in Los Angeles; hosted a fundraiser with about 160 people who bought tickets starting at $1,000; spoke at a rally for four northern California Democrats in tight House races; was in Nevada for a rally for Obama and Democratic Senate candidate Shelley Berkley; was at an Arizona State University rally with Democratic Senate candidate Richard Carmona; then came stops in Iowa and Indiana for more rallies with Senate and congressional candidates as well as fundraising events.
He's expected to keep up the pace until Election Day, Nov. 6.
All told, Clinton has appeared at no fewer than 32 fundraisers and 15 rallies for Obama. One big-ticket reception at a private New York City residence with Clinton last June drew 47 people who paid $40,000 apiece.
During a recent Las Vegas rally for Obama and other Democratic candidates, Clinton showcased his ability to poke fun at foes without demonizing them. He mocked Romney's move to the political center in the opening presidential debate.
"I had a different reaction to that first debate than a lot of people did," Clinton told the crowd. "I thought, 'Wow, here's old moderate Mitt. Where ya been, boy?'"
Clinton is also building favor with local Democrats, many of whom are longtime allies.
Cullen said Clinton's visit earlier this year to New Hampshire to endorse Democratic gubernatorial hopeful Maggie Hassan, a Clinton ally in 2008, provided a stamp of party establishment approval that helped Hassan win her competitive primary.
Yet Clinton's prominent campaign role could pose some potential downsides for Democrats. Some Democrats fret Clinton could overshadow Obama. A Pew Research Center poll last month showed 29 percent of those surveyed said Clinton's convention speech was the highlight of the party gathering, while just 16 percent called Obama's speech the highlight.
But there's no hiding the Clinton magic during an election year.
In Indianapolis on Friday, the former president noted that he had stayed the night before in Washington with Hillary, celebrating their 37th wedding anniversary. He recalled that she told him she wished she could come with him to campaign this year.
One of the event organizers had apologized to the former president for the size of the crowd, saying "that's as good as we can do for a crowd here except for a basketball game."
Clinton, famously fond of a crowd of any size, grinned.
"I'll take that," he said.