A Joe Biden Joe BidenOmar fires back at Trump over rally remarks: 'This is my country' Trump mocks Biden appearance, mask use ahead of first debate Trump attacks Omar for criticizing US: 'How did you do where you came from?' MORE presidential campaign would help Bernie Sanders Bernie SandersButtigieg stands in as Pence for Harris's debate practice Bernie Sanders warns of 'nightmare scenario' if Trump refuses election results Harris joins women's voter mobilization event also featuring Pelosi, Gloria Steinem, Jane Fonda MORE by hurting Hillary Clinton Hillary Diane Rodham ClintonButtigieg stands in as Pence for Harris's debate practice Senate GOP sees early Supreme Court vote as political booster shot Poll: 51 percent of voters want to abolish the electoral college MORE, according to Democratic Party insiders and other experts.

Biden, whose ideology is more similar to Clinton’s than the left-wing senator from Vermont, would siphon off more of her supporters, according to most polls.

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He could also help Sanders by turning the Democratic fight into a three-horse race.

“He should be leading the ‘Run, Joe, Run’ campaign,” said Joe Trippi, the Democratic strategist who ran former Vermont Gov. Howard Dean’s 2004 presidential campaign.

There is deep skepticism in Democratic circles that Sanders can expand his national support much beyond his current high-water mark of around 35 percent.

With Biden in the race, however, that’s less of a problem.

“If you make the race a 33-33-33 jump ball, then Sanders has a shot to tip that ball, and so does Joe,” said Trippi. “Whereas when you see this race with just Sanders and [Clinton], it goes 55-33, or whatever.”

Some pundits have argued that Democratic voters can be broadly divided into pro-Clinton and anti-Clinton camps, and that Sanders and Biden would hurt each other by splitting the votes of the dissenters.

But that theory does not really hold water, according to Tom Jensen, the director of the Democratic-leaning Public Policy Polling firm.

“We generally find that supporters of Biden or Sanders like Hillary Clinton, they just like one of those two more,” he said. “It’s not really a disliking-Hillary issue. Both Biden and Sanders have people who really like them.”

Tad Devine, a senior adviser to Sanders, backed up that assessment.

“I don’t see the Bernie Sanders vote as being an anti-Hillary Clinton vote, I really don’t,” he insisted. “I’m sure there are some people who support him who don’t like her. But if you went to one of his events, people will say nice things about her… Whenever this question has been asked [in polls] — ‘Are you voting for him or against her?’ — it has been 90-10 that they are voting for him.”

Polling at this stage of a presidential race ought to carry a health warning.

That’s especially true of findings related to Biden, who has not yet entered the race and may never do so. Still, the data does indicate that a run by the vice president would at least narrow Clinton’s lead.

An NBC News/Wall Street Journal poll earlier this week gave Clinton a 15-point lead over Sanders, 53 percent to 38 percent, without Biden in the race. With Biden in, her lead shrank to 7 points. Under the second scenario, Clinton took 42 percent, Sanders 35 percent and Biden 17 percent.

A CBS News poll earlier in September had better figures for Clinton overall but a similar pattern. It found that Sanders’ support would decline by just a single percentage point, from 28 percent to 27 percent, if the vice president ran; Clinton’s would fall by 11 points, from 58 percent to 47 percent.

“There is definitely an anti-establishment vote out there,” said Democratic strategist Brad Bannon, “What Biden is doing is taking some of the establishment vote away from Hillary.”

If Biden enters the race, the Sanders campaign is likely to argue he and the former secretary of State are cut from similar cloth.

“I think if you start to get into the voting records, you are going to see a lot of similarities,” said Devine, the Sanders adviser.

Biden backed the invasion of Iraq and is perceived as close to the credit card industry because of his long career representing Delaware in the Senate.

Devine also suggested Biden would pose “immediate competition” for Clinton in the battle for establishment support among “voters, fundraisers or super-delegates.”