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Will Joe Biden challenge Hillary Clinton for Democratic nomination?

Vice President Biden has not closed the door on a presidential run, according to NBC News sources.

Vice President Biden has not closed the door on a presidential run, according to NBC News sources — but it is far from clear whether he will enter the 2016 campaign, with much of the Democratic Party already behind Hillary Clinton.

These sources confirmed a New York Times report published Saturday that suggested Biden could still enter the presidential race, and that Biden allies have talked to party activists about the viability of a campaign.

The Times account suggested Biden's late son, Beau, had pushed his father to wage a third campaign for the White House.

Related: What would bring Biden into the 2016 race?

In a statement, Biden's press secretary Kendra Barkoff did not rule out a Biden campaign, instead sidestepping the issue.

"As the Biden family continues to go through this difficult time, the Vice President is focused on his family and immersed in his work," Barkoff said. "In recent weeks, the Vice President has worked on the nuclear deal with Iran, traveled across the country to highlight the administration's economic priorities and more."

The challenges for Biden to enter the race would be considerable. Hillary Clinton has raised more than $45 million already, while Biden struggled to raise funds in 2008, when he ran for president but didn't win a single state.

A huge bloc of Democratic senators, House members and governors have already endorsed Clinton.

Some Democrats, according to NBC News sources, have told Steve Ricchetti, Biden's chief of staff, that they wish Biden had signaled earlier that he wanted to run. Much of the party's establishment, assuming Clinton was running and Biden was not, have already committed to the former first lady.

Even some of Biden's former top aides have taken steps to back Clinton, suggesting they were not anticipating a campaign from their ex-boss.

Ron Klain, Biden's former chief of staff, is expected to help Clinton prepare for the debates, while Tony Blinken, a longtime Biden adviser on foreign policy, donated to Clinton during the last quarter, as first reported by Vox.

Vermont Sen. Bernie Sanders has for now consolidated much of the anti-Clinton vote in the party, too.

"There's no hunger for him," said former Obama administration aide Van Jones, in an interview on CNN on Sunday, discussing Biden's candidacy.

Jones added: "He is incredibly respected. He's a beloved figure in our party. But the reality is he's not so different from Hillary Clinton, from the point of view of the rising energy in the party. The Clinton party was a very pro-incarceration party, anti-regulation for Wall Street, and being on triangulation.

"The new party — the Elizabeth Warren wing, Bernie Sanders, Black Lives Matter, less incarceration, more regulation, and they hate the triangulation — they want authenticity. So, he checks the authenticity box, but no other box in our party."

And yet Biden allies say some Democrats have reached out to the vice president's team, expressing concerns that Clinton could be a weak candidate in the general election, amid the controversies over her use of a private e-mail account as secretary of state and the Clinton Foundation accepting money from foreign governments.

Related: What a Biden campaign would mean for Hillary Clinton

"He is definitely thinking about it. Not just for show," said one former senior Biden aide, who did not want to be quoted publicly. "And there are many people urging him to do it. And some at the very top. The money is going to be challenging, yet all he needs to do is pull close in Iowa or (New Hampshire)."

"All in all, definitely doable, yet at the end of the day I think he will decide to take a pass and instead spend time with family," this former aide said.

Biden allies have suggested there is a groundswell of support for him, but little of that has emerged so far publicly. Many prominent liberal activists were clamoring for Massachusetts Sen. Elizabeth Warren to run earlier this year, and the "Draft Warren" movement drew much more support than a similar "Draft Biden" push has.

Biden is known to have many allies on Capitol Hill from his more than three decades in the Senate — but 85 Democratic House members and 30 senators have already endorsed Clinton, according to The Hill.

And many of these lawmakers, such as Minnesota's Al Franken, endorsed Clinton before she was an actual candidate, bypassing the sitting vice-president for the former secretary of state.

NBC's Michael LaRosa, Andrea Mitchell and Kristin Welker contributed to this story.

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