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's electability argument is increasingly showing signs of weakness.

A Monmouth University national poll released Monday found Biden falling by 13 points and in a three-way tie nationally with Sens. 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.) and 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.).

Democrats say it’s the latest piece of evidence that the former vice president can’t expect to win simply by running on the idea that he is the candidate best positioned 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 in a general election.

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Voters in the Democratic Party are clearly focused on nominating a candidate who can win in November, and this has long been seen as a reason Biden has been riding high in the polls — in addition to his strong name identification.

But other candidates are making the argument that they, not Biden, are the nominees most likely to defeat Trump.

Furthermore, Democratic strategists say Biden has to give voters other reasons to back him too.

“It’s a slippery slip 'n slide if the one argument he’s basing his candidacy on falls apart,” said Democratic strategist Michael Trujillo, who worked on Hillary Clinton Hillary Diane Rodham ClintonBloomberg rolls out M ad buy to boost Biden in Florida Hillicon Valley: Productivity, fatigue, cybersecurity emerge as top concerns amid pandemic | Facebook critics launch alternative oversight board | Google to temporarily bar election ads after polls close Trump pledges to make Juneteenth a federal holiday, designate KKK a terrorist group in pitch to Black voters MORE’s 2008 presidential campaign.

“Is this the beginning of the house of cards coming apart? Maybe,” he said when asked about the Monmouth poll. If it is correct, he said this is “exactly where the Biden campaign does not want to be.”

The Monmouth poll comes just before Labor Day, when the Democratic campaign season intensifies and the candidates move into the heart of the primary election cycle.

“I hope the Biden folks are smart enough to realize that they can’t run solely on the electability argument because I’m pretty confident that won’t work out in the long run,” said Democratic strategist Jim Manley.

“Everyone is going to throw a lot more punches from here on out, and it won’t be easy for Biden to run on electability alone,” he said. “It’s going to get a lot harder.”

While electability has been the central premise of the campaign, it isn’t the only argument Biden has made for his candidacy.

He’s sought to portray himself as a unifier at a tumultuous time for the country.

“The president has decided to be president for his base. I will be a president for every American,” Biden said this weekend at Keene State College in New Hampshire. “People know who Donald Trump is. We need to let them know who we are.”

After the Monmouth poll was released on Monday, Biden aides called the survey “an outlier,” pointing to the low sample size of those who were questioned — 298 people.

Other Biden allies pointed to a CNN poll out last week that showed the former vice president with a double-digit lead. That survey showed Biden with 29 percent and Sanders and Warren at 15 percent and 14 percent, respectively.

A Quinnipiac University poll out earlier this month also showed Biden as the sole front-runner with 32 percent compared to 21 percent for Warren and 14 percent for Sanders.

“The poll is a stretch,” one Biden ally said of the Monmouth poll. “Every other poll has us in the lead by a long shot.”

But Democrats say that Biden’s reliance on “electability” is proving to be a faulty one, particularly as Biden continues to make verbal flubs. This weekend, for example, he said he was in Vermont when he was in New Hampshire.

“Biden’s gaffes on the campaign trail have undermined his own electability argument,” said Brad Bannon, a Democratic strategist. “Primary voters see, hear or read about the gaffes and question whether the former vice president can stand up to the beating he would get from Trump next year.”

“Biden is making unforced errors, which would come even more frequently next year if he’s the nominee and Trump pummels him relentlessly,” he added.

A recent Fox News poll showed that other candidates would also beat Trump in a hypothetical match-up. While Biden would best Trump by 12 points, according to the survey, Sanders would beat him by 9 and Warren by 7. Sen. Kamala Harris Kamala HarrisTexas Democratic official urges Biden to visit state: 'I thought he had his own plane' The Hill's Campaign Report: Biden on Trump: 'He'll leave' l GOP laywers brush off Trump's election remarks l Obama's endorsements A game theorist's advice to President Trump on filling the Supreme Court seat MORE (D-Calif.) would also defeat him by 6 points.

“As these other candidates build their name ID, you’re seeing their numbers match Trump’s or exceed Trump’s,” Trujillo said, adding that Biden is “basing his argument on something that will erode, even slowly.”

But Team Biden keeps returning to the electability theme. Last week, Jill Biden, the former second lady, pushed the electability factor during a campaign event with fellow educators in Manchester, N.H.

“I know that not all of you are committed to my husband, and I respect that, but I want you to think about your candidate, his or her electability, and who’s going to win this race,” she said. “You know you may like another candidate better, but you have to look at who’s going to win.”

“Joe is that person,” she added.

Manley said the Monmouth poll at the very least should serve as a wake-up call about the former vice president’s strategy.

“They’ve still got a lot of work to do,” he said. “They’ve got a lot more selling to do.”