Allies to Joe Biden Joe BidenFormer Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick Bloomberg rolls out M ad buy to boost Biden in Florida MORE worry that the electability argument the former vice president has made central to his campaign is losing its luster.

One longtime Democratic donor who has contributed to the former vice president’s campaign and once believed he was the only candidate formidable enough to defeat President Trump Donald John TrumpFederal prosecutor speaks out, says Barr 'has brought shame' on Justice Dept. Former Pence aide: White House staffers discussed Trump refusing to leave office Progressive group buys domain name of Trump's No. 1 Supreme Court pick MORE said Sen. Elizabeth Warren Elizabeth WarrenOvernight Defense: Appeals court revives House lawsuit against military funding for border wall | Dems push for limits on transferring military gear to police | Lawmakers ask for IG probe into Pentagon's use of COVID-19 funds On The Money: Half of states deplete funds for Trump's 0 unemployment expansion | EU appealing ruling in Apple tax case | House Democrats include more aid for airlines in coronavirus package Warren, Khanna request IG investigation into Pentagon's use of coronavirus funds MORE (D-Mass.) is making a large dent in Biden’s argument.

The donor said people in Biden’s orbit see Warren as a real threat and are acknowledging her steady progress in the race.

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“She’s rocking and rolling,” the donor said. “She’s taking from [Sen.] Kamala [Harris (D-Calif.)], she’s taking from Bernie [Sanders] and she could eventually take from Biden.”

A Biden supporter who worked as an aide to former President Obama said Biden can’t just argue he’s the most electable Democrat if he hopes to fend off the challenge from Warren.

“He needs to offer more. He can’t simply say, ‘I’m the only one who can beat Trump,’ ” the ex-Obama aide said.

An NBC News–Wall Street Journal poll released Tuesday found that Biden remains the Democratic front-runner with 31 percent, a 5-point rise from the last poll in July. But even that poll showed a small gain for Warren, who jumped 6 points to 25 percent.

Warren, whose campaign boasted of attracting 20,000 people to a rally in lower Manhattan earlier this week, has enjoyed a steady rise through the summer while rolling out a string of policy proposals.

She has also been highlighting her own electability, hammering home the idea that she can also beat Trump.

Speaking at the Massachusetts Democratic Convention earlier this week, Warren sought to prove this by going directly after Trump while highlighting her “big ideas” proposals.

“This dark moment requires more than being ‘not Trump,’ because a country that elects someone like Donald Trump is a country that’s already in serious trouble,” Warren said. “We need to talk honestly about what’s broken in America, but even more than that, we must show America that we have plans to make big structural changes to fix what’s broken.”

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Even Democrats who aren’t tied to any particular candidate say Biden has to offer a central rationale for his candidacy beyond electability to win.

“Both Elizabeth Warren and Joe Biden are polling well,” said Robert Zimmerman, a Democratic National Committee member and Democratic donor. “Warren’s bold and courageous ideas have created a mission and momentum for her candidacy. Biden, who would be an excellent president, has to create a mission for his campaign, not just rely on polling numbers for momentum.”

Biden allies say they feel good about their current standing.

They point to the diversity of their coalition, reflected in the NBC–Wall Street Journal poll.

While Warren has been able to lure educated white progressives to her campaign, she is well behind him in attracting black voters, a bulwark of any Democratic candidacy.

The new poll shows her support rising among black voters to 13 percent, well beyond the 49 percent for Biden.

“Joe Biden is building a coalition and you can’t say the same for Warren,” said one longtime Biden aide. “His campaign looks like this country. He’s reaching out to all corners of the Democratic Party. She’s not going to be able to win the nomination on the path she’s taking.”

Biden aides and advisers also point to other polls that show Biden leading in states like California.

A SurveyUSA poll released this week showed that more voters are confident that Biden can defeat Trump than say Warren will beat the president. The poll showed that 61 percent of those surveyed say Biden can beat Trump, compared to 41 percent for Warren and Sen. Bernie Sanders Bernie SandersSirota reacts to report of harassment, doxing by Harris supporters Republicans not immune to the malady that hobbled Democrats The Hill's Morning Report - Sponsored by Facebook - Republicans lawmakers rebuke Trump on election MORE (I-Vt.).

Biden’s camp insists he isn’t underestimating Warren and acknowledges the Massachusetts senator is setting herself up to be a strong competitor. But they suggest Biden doesn’t need to react to her progress so much as his team must focus on running their own operation, while advancing the former vice president’s rationale for running.

Still, even staunch Biden supporters worry that Warren has enthusiasm on her side. The NBC–Wall Street Journal poll showed that 70 percent of Democratic primary voters said they were “enthusiastic about or comfortable with” Warren. Sixty-four percent felt the same about Biden.

Biden has continued to lean into his time as Obama’s vice president, invoking his former boss during debates, campaign ads and rallies. The effort is intended to keep Biden in the hearts of Obama’s most loyal supporters.

“He’s not going to be the Energizer bunny when it comes to exciting Democrats or playing the crowd count game,” said Michael Trujillo, a Democratic strategist. “The way he wins is to transfer as much of the Obama loyalty as possible.”

Democratic strategist Basil Smikle said Biden also needs to add more of his own touch to the campaign.

“I generally think he needs to go on the offensive more — in terms of policy proposals, vision for the country, support for constituencies,” Smikle said. “That’s the key to Warren’s surge. Doing so will ultimately draw distinctions between himself and others.”

Still, he added, “As long as he leads in the polls, Biden’s argument still holds even while another path to the nomination is being blazed by Warren.”