Former Vice President Joe Biden Joe BidenBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Democratic groups using Bloomberg money to launch M in Spanish language ads in Florida Harris faces pivotal moment with Supreme Court battle MORE’s looming entry into the Democratic presidential primary sets up what could be a titanic clash against Sen. Bernie Sanders Bernie SandersOutrage erupts over Breonna Taylor grand jury ruling Dimon: Wealth tax 'almost impossible to do' Grand jury charges no officers in Breonna Taylor death MORE (I-Vt.).

The elder statesmen who are the two front-runners for the nomination served together in the Senate briefly. Biden was the far bigger star than Sanders on Capitol Hill.

But Sanders has been more of a liberal policy visionary, championing policies that were initially regarded by colleagues as fringe ideas. These concepts were later embraced by leading members of the Democratic caucus, such as raising the minimum wage to $15 an hour and providing free college education.

Former colleagues describe Biden and Sanders as polar opposites in terms of personalities and interacting with fellow senators.

Biden, 76, is gregarious and sociable. He was always eager to work a room, strike up a friendly conversation or reassure a colleague who was feeling down. A retail politician to his core, Biden always wanted to know what was on his fellow senators’ minds.

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Sanders, by contrast, was focused on policy, often so wrapped up in his thoughts that he seemed oblivious to his colleagues. He was seen as consumed by ambitious plans to fight wealth inequality and push the national debate to the left but uninterested in the personal lives of fellow senators.

“Joe and Bernie are very different people,” former Sen. Byron Dorgan (D-N.D.) said with a chuckle. “Joe is very outgoing, very social.”

“Bernie is not that. Bernie has a different kind of personality. If the two of them were walking into a room together, Joe’s going to walk in with his hand out and begin meeting everybody. Bernie has a different approach,” he added.

Yet one Sanders ally said that while Sanders wasn’t chummy with his fellow senators, he liked Biden and considered him a friend.

Biden swore Sanders into office in 2007. At the time, the two men exchanged pleasantries, with Biden wishing Sanders’s wife, Jane, happy birthday.

Sanders, 77, met with Biden before the 2016 presidential election as he was mulling a challenge to Hillary Clinton Hillary Diane Rodham ClintonDemocratic groups using Bloomberg money to launch M in Spanish language ads in Florida The Hill's Campaign Report: Presidential polls tighten weeks out from Election Day More than 50 Latino faith leaders endorse Biden MORE, who was the overwhelming favorite to win the Democratic nomination, to gauge Biden’s thoughts on the potential race. Biden subsequently revealed that he counseled Sanders not to accept money from super PACs.

“I sat with Bernie,” Biden said of the meeting. “I’m the guy that told him, you shouldn’t accept any money from a super PAC, because people can’t possibly trust you. How will a middle-class guy accept [you] if you accept money?”

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It remains to be seen if Biden, who is expected to announce his bid on Thursday, will accept donations from super PACs.

On another occasion early in Sanders’s Senate career, Biden urged Sanders to be a little sunnier in his political speeches, which up to that point had tended to be heavy with gloomy rhetoric, according to a person familiar with their relationship.

Sanders admits he doesn’t love schmoozing.

“Am I a grump? Yeah,” he told CNN’s Gloria Borger in 2015. “If the question is, am I much of a small-talker, or am I a good schmoozer … I’m not so great at that.”

In one early Senate interaction, Biden chatted up Sanders at length, prompting the business-like Sanders to later observe privately to a colleague that it could be difficult to extract oneself out of a conversation with the future vice president once he got rolling.

A second Sanders ally said any feelings of friendship that might have existed between Sanders and Biden are likely to go out the window once Biden jumps in the race.

“They don’t have any kind of relationship,” the source said. “There’s no love lost there. I’d be shocked if they’ve spoken [recently].”

The Sanders ally added that Biden has “unthinkable seniority in that Senate. Bernie was never in that club.”

“Bernie would say Biden is part of the Democratic establishment that we need to fix,” the source said.

The Sanders ally added that both men “will inevitably go at each other. Bernie will go at him on issues because that’s all he cares about. I think that will be the glaring difference.”

Biden was elected in 1972 to the Senate and left in 2009 to serve as former President Obama’s vice president. Sanders was in the House from 1991 to 2007 before being elected to the Senate. They served together in the upper chamber for two years during a new Democratic majority.

“I don’t know that I saw a lot of interaction,” former Sen. Ben Nelson (D-Neb.) said of the relationship between Biden and Sanders, describing Sanders as “sort of a loner in the Senate.”

By contrast, Biden loved to gab, Nelson said. He recalled a time when Biden was vice president and invited him to the White House for an afternoon to sip lemonade and munch on cookies.

Biden’s top priorities in the Senate were his assignments on the Senate Judiciary and Foreign Relations committees, while Sanders was more focused on his top committee assignments: the Health, Education, Labor and Pensions Committee, the Environment and Public Works Committee and the Budget panel.

Sanders’s focused ambition led colleagues to suspect he might someday run for president.

A Democratic strategist said there was a concern in Obama circles that Sanders could run in 2012 because he thought Obama wasn’t progressive enough.

Sanders allies admit that Biden is their biggest worry. For starters, they are both trying to appeal to independent voters and also disenfranchised Republicans.

“I think from that side of it, Bernie doesn’t like Biden,” the strategist said, adding that the theory around Democratic circles is that “Bernie’s camp is the source of much of the Biden opposition being dropped.”

“So I wonder if Bernie goes on the attack early and — which then makes Biden have to choose [to] ignore the attacks and do his own thing or get down in the mud and kick Bernie’s ass,” the strategist said.

In the 2016 Democratic primary, Sanders vowed not to go negative against Clinton. Regardless, bad blood remains between the camps of Sanders and Clinton.

The biggest face-to-face policy clash between Biden and Sanders came in 2010 after Biden and then-Senate Minority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellFEC flags McConnell campaign over suspected accounting errors Poll: 59 percent think president elected in November should name next Supreme Court justice Mark Kelly: Arizona Senate race winner should be sworn in 'promptly' MORE (R-Ky.) cut a deal to extend all of the Bush-era tax cuts — except for the elimination of the estate tax — for two years. Biden and McConnell put the deal together in secret talks after Republicans wiped out Democrats in the previous month’s midterm election and won back control of the House.

Colleagues said Sanders was irate when Biden showed up at a Senate Democratic lunch meeting to sell skeptical senators on the need to extend all of the Bush tax cuts to avoid tax breaks for middle-class Americans lapsing before Obama’s 2012 reelection campaign.

“I do remember Sanders was very exercised, very opposed,” said former Sen. Kent Conrad (D-N.D.). “It was a very contentious [meeting].”

“I especially remember Sanders was very upset,” he added.