The top Democrat in the Senate, Chuck Schumer of New York, is promising to block one of President-elect Donald Trump’s first big initiatives — naming a ninth member to the Supreme Court.

“It’s hard for me to imagine a nominee that Donald Trump would choose that would get Republican support that we could support,” Schumer said in an interview Tuesday night on MSNBC’s “The Rachel Maddow Show.”

Asked whether he’ll do his “best to hold the seat open” on the Supreme Court, Schumer responded, “Absolutely.”

For Schumer, it’s about retribution. The Republican-controlled Senate failed even to vote on President Obama’s last nomination to the highest court, Merrick Garland, who was put up for the job after the sudden death of Justice Antonin Scalia.

Republicans instead made the Supreme Court a campaign issue, saying whoever was elected president would get to nominate Scalia’s replacement.

The Democratic Senate leader told host Maddow that Republicans got away with not voting on Obama’s nominee, but that “the consequences will be down the road.”

But in June, Schumer sang a different tune, blasting Republicans for not doing their duty and for creating “chaos.”

“Senate Republicans need to do their job and give Judge Garland the hearing and vote he deserves because the American people deserve a fully functioning court,” said Schumer in a June 8, 2016, press release.

“Having a deadlocked, 4-4 court could lead to judicial chaos surrounding environmental protections, voting rights, and so many other issues that are important to everyday Americans. This delay has gone on long enough, it’s time for the Senate to do the job we were elected to do,” he added.

But a spokesman for Schumer said his boss’ opposition to Trump’s Supreme Court pick will only be if the president-elect ventures “out-of-the mainstream.”

“The Senator was referencing a scenario in which President-elect Trump and the Republicans nominate someone who is out-of-the-mainstream, one whose views don’t align with the majority of the American people,” said spokesman Angelo Roefaro in an email to The Post.