WASHINGTON — Since Joe Biden ran away with the Democratic presidential nomination in March, leading progressives have accepted him – sometimes grudgingly – as their party’s leader. But, in the final weeks of the campaign, the Supreme Court vacancy is threatening to inflame old divides.

Some activists on the left are pressing Biden to endorse expanding the number of high court justices should he win the White House and Democrats take control of the Senate. But Biden, who ran a relatively centrist primary campaign, hasn’t embraced those calls, worried they may intensify the nation’s partisan split.

There’s little indication that large swaths of progressives will abandon Biden or back third-party candidates, moves that wounded Democrat Hillary Clinton’s 2016 bid. But activists insist they will keep pressure on Biden to pursue dramatic reforms to the Supreme Court if Republicans move forward with a plan to quickly approve President Trump’s pick to replace the late Justice Ruth Bader Ginsburg.

“The majority of Berniecrats will most likely vote for Vice President Joe Biden,” said Nina Turner, a former Ohio state senator and top adviser to progressive Sen. Bernie Sanders’ 2016 and 2020 presidential campaigns.

“That doesn’t mean that they are not going to raise hell all the way.”

“Biden should make it clear that he will fight back by expanding the court if he wins,” said Turner, who is founding a firm to advance progressive causes, Amare Public Affairs.

The Constitution doesn’t mandate the number of Supreme Court justices, which has changed over time. In 1937, President Franklin D.

Roosevelt promoted legislation to “pack the court” by expanding its number of justices, an effort that stalled once the justices began to rule in his favor on policies tied to the New Deal.

Since then, the makeup of the court hasn’t been a prominent issue in national politics. That began to change after Justice Brett Kavanaugh’s contentious 2018 nomination fight. Calls to add justices grew much louder this week in response to the GOP’s rush to fill Ginsburg’s seat before the election, which would leave the court with six conservatives and three liberals.

“The politics of this are moving very, very fast,” said Aaron Belkin, director of Take Back the Court, which advocates for increasing the number of justices. “And under a Biden administration, when the court has the administration handcuffed on Day One, I think the politics are going to be changing even more quickly.”

That puts Biden in a tough spot. As someone who spent 36 years in the Senate, he built a career revering Washington’s institutions.

During the 2020 primary, he pointedly declined to join rivals such as Massachusetts Sen. Elizabeth Warren or California Sen. Kamala Harris, who is now his running mate, in being open to court expansion.

During his first extended comments Sunday about Ginsburg’s death, Biden appealed to the few remaining moderate Senate Republicans to buck their party’s leadership, rather than to progressives looking for him to support a larger court.

Biden has also said Democrats should concentrate on making it clear for voters why the GOP push to quickly fill Ginsburg’s seat is a “gigantic mistake and abuse of power.”

Biden should make it clear that he will fight back by expanding the court if he wins.” NINA TURNER Adviser to Sanders campaigns