With the two candidates leading the Democratic presidential race, left-leaning voters could soon be forced to choose between two very different tracks: the progressive path promised by Sanders or the more moderate course championed by Biden.

However, with Super Tuesday exit polls painting a clearer picture of who is backing who in the Democratic primary race, experts have warned that if Sanders wants to go head-to-head with Biden, he may need to “expand his political base”—and fast.

Whereas Biden’s popularity appeared to be relatively even between men and women voters, with the former vice president seeing more support among the latter group in some states, according to Super Tuesday exit polls, Sanders appeared to enjoy more support from men than women.

According to exit poll results published by The Washington Post, Sanders saw more support from men than women in every Super Tuesday state and territory: Alabama, California, Colorado, Maine, Massachusetts, Minnesota, North Carolina, Oklahoma, Tennessee, Texas, Virginia and in Vermont, which appeared to see the smallest disparity between support from men and women.

Moving forward, Professor Peter Trubowitz, the Director of the U.S. Centre at the London School of Economics (LSE), told Newsweek on Wednesday: “Sanders’ big problem going forward is to expand his political base.”

“All of these campaigns are run at the margin,” Trubowitz said. “If you see that there is a 10-point disparity in female voters, you have to figure out where you’re going to get those female voters.”

Kelly Dittmar, an assistant professor of political science at Rutgers University–Camden and scholar at the Center for American Women and Politics at the Eagleton Institute of Politics, agreed, telling Newsweek: “It’s safe to say [Sanders] has a pretty consistent gender gap across these states where he performs better among men than women.”

Vermont, where Sanders enjoys widespread support, she noted, was the only state “where you really saw him close that difference between men and women.”

“This is really important,” Dittmar said, because with women comprising about 60 percent of Democratic primary voters and typically turning out at higher rates than men, their votes are not ones Sanders will want to lose.

“When we look at these numbers in isolation, you can say, ‘well, it balances out: Biden does better with women, Bernie does better with men,’ but it’s not the same,” Dittmar said.

With more primaries on the horizon, Trubowitz said Sanders and his surrogates could start looking to draw more support away from a close competitor: Massachusetts Sen. Elizabeth Warren.

“After the results that came in yesterday, as a result, he, or, more likely, his surrogates, could put pressure on Elizabeth Warren to step aside,” Trubowitz warned.

If they do, he said, it will be based on the “assumption that a disproportionate number of [her] voters will be pushed in his direction.”

It is unclear whether Sanders or his so-called surrogates would consider pressuring Warren to drop out of the race, particularly since he faced similar pressures himself in the 2016 Democratic presidential race before ultimately acknowledging Hillary Clinton as the victor in the nominating process and giving the former Secretary of State his endorsement.

In that race, while Clinton saw more support from women than Sanders overall, it was the Vermont Senator who appeared to be the preferred candidate of the two among millennial women.

Dittmar said she hoped Sanders would not look to pressure Warren to drop out of the primary race for his own benefit. That decision, she said, should be up to candidates themselves.

“I think it’s up to her,” the associate professor said of Warren, who failed to win her own state, losing out to both Biden and Sanders. “She has invested a lot of time and resources and she has generated an incredible amount of support nationwide…including women who are particularly energized around her campaign.”

“Telling candidates that they have to drop out as soon as they lose a couple of primaries is not consistent with what the primary process is actually meant to do,” Dittmar asserted.

Warren, she said, “is smart. She’s going to look at whether or not she can sustain this candidacy and whether or not she sees a path to victory and she’ll make that choice.”

Newsweek has contacted the Sanders and Warren campaign teams for comment.