WASHINGTON -- Over the past quarter century or so, Hillary Rodham Clinton and Joe Biden have collaborated and competed, shared more than a dozen staffers, fought each other for a presidential nomination and then served together in the Cabinet of the man who beat them both.
Now, as Biden considers challenging Clinton for the 2016 Democratic presidential nomination, their long and tangled relationship is being tested anew. Both now find themselves elder statesmen in the Democratic Party -- Biden is 72, Clinton 67 -- and facing similar questions about whether they represent the party's future or its past.
They've been linked in presidential politics before, and not just in the 2008 campaign when they lost the Democratic nomination to then-candidate Barack Obama. In late 2011, when Obama's re-election looked uncertain, his advisers secretly polled whether they should consider replacing the vice president with Clinton, who was then secretary of state.
Four years later, it's Clinton's candidacy that is stressed, leading some prominent Democrats to question whether Biden would make the stronger candidate. Yet despite the prospect of a primary faceoff, aides and colleagues say their relationship remains relatively warm.
"They have a genuine friendship," said former Sen. Bob Kerrey, D-Neb., who has known both Democrats for decades. "If Biden becomes a candidate, you could probably put the sun and the moon between them. But it's not going to be because he lacks respect or admiration for Hillary Clinton."
In June, Clinton flew to Delaware for the funeral Mass for Biden's son Beau, along with former President Bill Clinton, who sat with her near the front of the church. But as mourners streamed out, Clinton left in a hurry.
She hopped a flight to Connecticut, barely making it in time for the wedding of former Biden national security adviser Jake Sullivan, whose decision to forgo a job on a potential Biden campaign in favor of the top policy job on Clinton's campaign served as another reminder that their rivalry is never far from the surface.
That many of Biden's top advisers have left his orbit to work for either Clinton or Obama remains a sore point for the vice president, said several of his aides, who requested anonymity because they weren't authorized to comment publicly. Sullivan, who had worked for Clinton previously at the State Department, made clear from the get-go that his first loyalty was to her, the aides said.
As talk of a Biden challenge grew louder this month, Clinton said in New Hampshire, "I have the highest regard and affection for him. We should all let the vice president be with his family and make whatever decision he believes is right."
Their alliance dates to Bill Clinton's presidency in the late 1990s, when Biden used his perch atop the Senate Judiciary Committee to help pass legislation targeting crime, gun violence and domestic abuse. Years later, when Hillary Clinton was preparing to enter the Senate, one of her first stops was Biden's office.
Biden's former Senate staffers say Clinton wasn't a frequent visitor in the subsequent years; they sat on separate committees and focused on different issues. But Biden kept a photograph in a prominent spot on his office bookshelf: Hillary Clinton, laughing wildly, with Biden whispering in her left ear while Bill Clinton whispered in her right.
Another moment between the two marked a notably thoughtful overture in the midst of the fierce 2008 presidential campaign, in which Biden and Clinton were slightly softer on each other than on their other opponents.
Obama had just bested Clinton, Biden and half a dozen others in the Iowa caucuses. Democratic lawmakers were eager to accompany the newest political superstar to the State of the Union address.
Not Biden. Days before the speech, he sent word, Clinton's former aides recall: Could he escort her past the cameras and crowds into the House chamber?
At the speech, Clinton sat defiant in a bright red suit -- Biden by her side.
"There's no woman like Hillary Clinton," Biden said in 2013 as they shared the stage at an awards ceremony.
But they haven't always agreed -- most notably on Iraq. When Clinton put forth a plan to cut off funding to Iraq's military, Biden argued that it would be counterproductive. Biden had advocated his own plan for a decentralized Iraq.
"I think it would be a disaster if it is her plan," he told ABC's Diane Sawyer in 2007.
In the Obama administration, they met near-weekly for breakfast at Biden's official residence. Biden, "always the gentleman," would meet Clinton at the car and walk her to a sunny nook off the porch, she recalled in her book, "Hard Choices."
When they faced off in the 2008 campaign, Biden made a point to say he wasn't running against Clinton -- he was "running to lead the free world." But if he chooses to run again in 2016, Clinton will undoubtedly be his biggest obstacle to winning the nomination.
"I don't know," Clinton communications director Jen Palmieri recently told reporters quizzing her about Biden's prospects. "He's gonna decide. He said he would decide."