They may eventually fight over the same pool of voters, but for now Sens. Elizabeth Warren and Bernard Sanders appear to be working toward the same goal of trying to take down the leading 2020 Democratic presidential candidate: former Vice President Joseph R. Biden.

Mr. Biden has soared in the polls since announcing his bid for the nomination, and the two senators have figured they can’t let him get too far ahead.

“Both of their strategies have to be to bring Biden back to the pack — to reel him in just as you would with a marathon runner who has broken out to the lead,” said David Paleologos, director of the Suffolk University Political Research Center. “Basically they are partners-in-crime now and they will be adversaries later.”

Ms. Warren has hit Mr. Biden up high, complaining he’s beholden to big-dollar interests and has been on the wrong side of key liberal agenda items such as bankruptcy laws that hurt consumers.

“At a time when the biggest financial institutions in this country were trying to put the squeeze on millions of hardworking families,” Ms. Warren said last month, “Joe Biden was on the side of the credit card companies.”

Mr. Sanders, meanwhile, has hit lower, saying he and the former Obama vice president have “very different pasts in terms of how we have voted and a very different vision for the future.”

The Vermont independent said when he was leading “the opposition against the Iraq War,” Mr. Biden was a vocal supporter. Same goes for civil rights and trade legislation, Mr. Sanders says.

“I voted against NAFTA, I voted against permanent normal trade relations with China — two trade agreements that cost us millions of good-paying jobs, Joe supported those agreements,” the senator said.

The Sanders and Warren campaigns did not respond to requests for comment.

Mr. Paleologos said Ms. Warren and Mr. Sanders are taking a calculated risk that neither of them will benefit more than the other.

“If you are in the single digits, you could raise awareness by going on the attack with Biden, but chances are statistically the beneficiaries would be Sanders or Warren because the person making the attack would have such low standing,” he said. “So Warren and Sanders say we are going to have to do it because it is going to benefit us.”

Their attacks contrast with Pete Buttigieg, the mayor of South Bend, Indiana, who’s polling near Ms. Warren and Mr. Sanders but who has passed on the chance to ding Mr. Biden.

Which strategy is more successful remains to be seen, but liberal activists are mimicking Ms. Warren and Mr. Sanders in saying Mr. Biden represents a Democratic Party of yesteryear.

The former vice president says he doesn’t see it.

“They talk about there is a division in the Democratic Party,” Mr. Biden said at a campaign stop in Iowa this week. “We agree on basically everything, all of us running — all 400 of us.”

Mr. Paleologos said Mr. Biden is portraying himself as the party’s safest bet to defeat President Trump in the general election, while Mr. Sanders and Mrs. Warren are arguing picking Mr. Biden would hardly be worth it for Democrats eager to break with the past.

The pollster said some of this is a race against time.

“When you think of the nine months between now and the caucuses you really have six weeks left of bona fide political news coverage until the summer when people tune out politics,” he said. “They have to do whatever damage that they can do sooner rather than later, and it will probably pick up again in August or September, but then it is a sprint at that point.”

South Carolina state Rep. Terry Alexander said he doesn’t see any downside to Mr. Sanders drawing distinction with Mr. Biden.

“Bernie has been setting himself apart for years,” Mr. Alexander, a Sanders supporter, said. “Any candidate would be foolish not to show their differences.”

Mr. Alexander said he expects the enthusiasm to start leveling off for Mr. Biden once the dust settles from his announcement and voters start giving his record another look.

Asked what he makes of the manner in which the liberal stars have characterized Mr. Biden, Rep. Ro Khanna of California, a Sanders backer, told The Washington Times that Ms. Warren and Mr. Sanders are “progressive icons.”

“We are lucky to have them both in the race, and I think they both are articulating a progressive vision and are passionate about making sure the party moves in a progressive direction,” Mr. Khanna said.

Norman Solomon, a Sanders delegate from California to the 2016 national convention, predicted Democrats will unify behind whoever wins the nomination, but said it is also important for voters to know that “Biden represents the corporate wing of the party — the wing that led us to defeat with Hillary Clinton in 2016. “

“So, it makes a lot of sense for Sanders and Warren to talk about how they’ve strongly disagreed with Biden on many crucial issues,” Mr. Solomon said. “Looking at his record and his tight mutual allegiances with moneyed elites, he’s not really ‘Lunch Bucket Joe,’ he’s ‘Wall Street Joe.’”

•Gabriella Munoz contributed to this report

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