WASHINGTON — Senate Republicans sent a letter to the Supreme Court on Thursday saying it “must not be cowed by the threats" from Democrats who urged the justices not to take up a gun case next term and warned that the court could be restructured to reduce the influence of politics in its decisions.

“Our colleagues did more than raise legal arguments in favor of mootness. They openly threatened this Court with political retribution if it failed to dismiss the petition as moot,” the Republicans said of the Second Amendment case, which involves a challenge to a now-defunct New York City law that barred the transport of firearms to gun ranges outside the city.

The letter, signed by Majority Leader Mitch McConnell of Kentucky and all the other Republican senators, pointed to proposals floated by 2020 Democratic presidential candidates to restructure the Supreme Court with the aim of making it more politically neutral.

Mayor Pete Buttigieg of South Bend, Indiana, for example, has proposed overhauling the court by expanding the number of justices from nine to 15, with five affiliated with Democrats, five with Republicans, and five apolitical justices chosen by the first 10. Others favor a rotating schedule of different justices on the bench.

“It’s one thing for politicians to peddle these ideas in Tweets or on the stump," the Republicans' letter said. "But the Democrats’ amicus brief demonstrates that their court-packing plans are more than mere pandering. They are a direct, immediate threat to the independence of the judiciary and the rights of all Americans.”

The Democratic amicus brief, filed by Sen. Sheldon Whitehouse of Rhode Island and four other Democratic senators, challenged whether the court’s conservative majority had become overly politicized and thus incapable of acting independently from interest groups such as the National Rifle Association and the Federalist Society.

The brief urged the court to drop the high-profile gun case out of New York, which it agreed to hear in its upcoming term this fall.

“The Supreme Court is not well. And the people know it,” the Democratic senators wrote. “Perhaps the Court can heal itself before the public demands it be ‘restructured in order to reduce the influence of politics.’ Particularly on the urgent issue of gun control, a nation desperately needs it to heal.”

Sens. Kirsten Gillibrand of New York, who dropped out of the presidential race Wednesday, Mazie Hirono of Hawaii, Richard Blumenthal of Connecticut and Richard Durbin of Illinois also signed the brief.

Democrats argue that the Supreme Court has become increasingly politicized, especially after the fraught confirmation fight over the nomination of Justice Brett Kavanaugh, as well as McConnell's earlier refusal to consider President Barack Obama's nominee, Merrick Garland, in 2016. They claim that restructuring the court and diluting the conservative majority is a necessary step to returning the court to the impartial judiciary it was intended to be.

Republicans, however, told the Supreme Court in their letter that no such change would happen under their watch. “While we remain Members of this body, the Democrats’ threat to 'restructure' the Court is an empty one," they wrote.