Washington (CNN) -- How things change.
In 2008, after a bruising primary battle with Hillary Clinton, then-Sen. Barack Obama was on the outs with former President Bill Clinton. This election cycle, Clinton is one of Obama's most visible surrogates.
Obama and Clinton were to appear together Monday night at three New York City fundraisers, which are expected to bring in millions of dollars. In April, the two appeared side by side at the home of longtime Clinton supporter and former Democratic National Committee Chairman Terry McAuliffe, raising $2.1 million for the Obama campaign and the DNC.
The Obama campaign is relying heavily on Clinton, tapping into his network of wealthy donors and featuring him in campaign ads. Clinton is also expected to hit the trail to rally voters leading up to the November election.
"Clinton just wants to help," a source close to the former president said. "His message is 'I know what it takes on the economy, and Obama is doing the right thing.' They're not going to make the mistake of the Gore campaign. They're not going to put [Clinton] on ice."
Obama's relationship with the Clintons has had its ups and downs. In 2007, the former president took aim at the then-junior senator from Illinois as inexperienced.
"You know, I'm old-fashioned. I think it really -- I think a president ought to have done something for other people and for his country when you pick a president," Clinton said. "I mean when is the last time we elected a president based on one year of service in the Senate before he started running?"
In early 2008, when Obama won the first-in-the-nation Iowa caucuses and emerged as a real contender, he entered into an all-out feud with the former first couple.
The tussle was on full display during a January 2008 debate.
"I'm here, not my husband," Hillary Clinton said to Obama.
"I can't tell who I'm running against sometimes," Obama retorted.
The Clinton source says, "He held a grudge, she didn't."
Obama picking Hillary Clinton as his secretary of state helped heal some wounds. And he's looked to Bill Clinton for help since taking office, hosting him at the White House during contentious negotiations with Congress in December 2010.
"I have a general rule, which is that whatever he asked me about my advice, and whatever I say, should become public only if he decides to make it public," Clinton told reporters then.
As Obama struggles to appeal to white, working-class males, he hopes Clinton can woo those voters in key Southern and Rust Belt states.
You wouldn't call the two men best of friends, but the lines of communication between the White House and the Clinton circle are open, sources in both camps say.
But even as the two are now on the same team, they're not always reading from the same playbook.
While the Obama campaign tries to make a liability of presumptive Republican nominee Mitt Romney's past as the head of a private equity firm, Clinton recently punched a hole in what has been a central campaign argument.
"I think he had a good business career," Clinton said of Romney last week on CNN's "Piers Morgan Tonight." "There's no question that in terms of getting up and going to the office and, you know, basically performing the essential functions of the office, the man who has been governor and had a sterling business career crosses the qualification threshold."
Clinton later clarified his remarks, but the damage was done.