WASHINGTON (NEXSTAR) — The prospect that President Donald Trump and Senate Republicans will fill Justice Ruth Bader Ginsburg’s Supreme Court seat before the year is out has ignited a call for major changes on the court, including expanding the number of justices.

The idea of expanding the high court has been labeled as “packing the court” in recent debates and analysis.

Here’s how the Washington Post defines packing the court:

Court packing is adding more judges to a court than there are now, something that can be done on the federal level simply by passing a law.

The Constitution says nothing about how many justices there must be on the Supreme Court, and over time, the number has fluctuated. The court started out with six justices, expanded to seven and has gone as high as 10. Congress set the Supreme Court to be nine justices in 1869, but if a president and Congress agree, they could change the law to expand the court or shrink it.

Some Democratic senators, who had been averse to increasing the size of the nine-member court, said in the wake of Ginsburg’s death that the Republican rush to fill the high court vacancy could be a breaking point.

But the sudden vacancy also is fueling tensions among Democrats who want presidential nominee Joe Biden to embrace reforms including adding justices to the court.

On Thursday, Biden said voters will “know my opinion on court packing when the election’s over.”

Biden has joined his party’s senators in calling for Republican Senate Majority Leader Mitch McConnell to delay a confirmation vote on Trump’s Supreme Court nominee, Amy Coney Barrett, until after the election. Barrett would give the court a 6-3 conservative majority.

Some progressives want Biden and Democrats to commit to expanding the court with a slate of liberal justices if they take power in January. Trump and Republicans are using that scenario in the hopes of animating the GOP base and perhaps coaxing votes from some moderate Republicans who dislike Trump but care about the court makeup.

Biden, who ran a relatively centrist primary campaign and spent 36 years in the Senate, is concerned that such moves would worsen divisions during a particularly polarized moment in American history.

“He believes in Senate institutions and he believes that the Senate did sort of function in a way that could can really kind of transcend politics,” AP national political reporter Will Weissert said.

Another worry is that changing the size of the court for the first time in 150 years would come back to bite the Democrats, leading to further expansion when Republicans next control both Congress and the White House. Faced with a 6-3 conservative court as the new year begins, Democrats would need to add four seats to overcome the Republicans’ edge. With a 15-justice court, just two more additions by the Republicans would solidify their advantage.

Democrats said almost nothing about the Supreme Court at their convention in August. That changed over the last month.

“Nothing is off the table” for Senate rules changes if Republicans quickly confirm a new justice, Senate Democratic leader Chuck Schumer of New York warned in a recent conference call.

If Democrats win control of the Senate in the November elections, they probably would need to get rid of the filibuster, the Senate rule that requires 60 votes rather than a simple majority for most legislation, before they could move on legislative changes that might include the size of the court.

The issue also came up in Wednesday’s vice presidential debate. Sen. Kamala Harris didn’t directly answer an accusation of supporting packing leveled by Vice President Mike Pence. Instead, she countered that the Trump administration has stacked federal courts nationwide with white conservatives, accusing Pence of participating in a different version of court packing.

“This is what they’ve been doing. You want to talk about packing the court. Let’s have that discussion,” she said. “Let’s go on and talk about the issue of racial justice.”

The Associated Press contributed to this report.