(Ricky Carioti/The Washington Post)

All 53 Republicans in the Senate urged the Supreme Court on Thursday not to be “cowed” by a brief from a handful of Democratic senators that warned that the court risks its reputation for impartiality if it continues to hear a gun case that the Democrats consider moot.

In a letter to the court’s clerk, the Republicans said the brief filed by Sen. Sheldon Whitehouse (D-R.I.) and four other Democrats “openly threatened this court with political retribution if it failed to dismiss the petition as moot.”

The justices “must not be cowed by the threats of opportunistic politicians,” said the letter, drafted by Senate Majority Leader Mitch McConnell (R-Ky.).

Whitehouse had told the court in a brief filed this month that because New York had changed the law in question, there was no longer the kind of live controversy that must be present for the court’s consideration. To hear the case anyway, he suggested, would reveal a bias.

“The Supreme Court is not well. And the people know it,” Whitehouse wrote in the brief. “Perhaps the Court can heal itself before the public demands it be ‘restructured in order to reduce the influence of politics.’ ” The phrase is from a poll question with which a majority of Americans agreed.

Whitehouse said the brief was meant to caution the court about the public’s perception of the institution.

“The response to our brief from Republicans and the partisan donor interests driving the court’s polarization shows exactly why it’s time to speak out,” he said in a statement Thursday. “They want us to shut up about their capture of the court; we will not.”

The partisan bickering has elevated what began as a somewhat obscure case about a law unique to New York City.

The Supreme Court earlier this year said it would hear New York State Rifle & Pistol Association v. City of New York in the term that begins in October. It concerns restrictions on how gun owners with residential permits may transport their weapons. The rules were so strict that they forbade taking an unloaded weapon to a firing range outside the city or to a permit-holder’s second home within the state.

But after the court accepted the case, the city surrendered. It rescinded the regulations that the gun groups and owners had objected to, and the state legislature passed a law prohibiting their reinstatement.

New York said the changes render the case moot, something the court is scheduled to consider Oct. 1.

The letter from the Senate Republicans says they are not taking a position on whether the case should be dropped or on the underlying question about Second Amendment protections. But it said those issues should be decided “as the law dictates, without regard to the identity of the parties or the politics of the moment.”

The letter notes that some Democrats in Congress and some of the party’s presidential candidates have said they would consider increasing the number of seats on the Supreme Court to dilute its conservative majority.

“The Democrats’ amicus brief demonstrates that their court-packing plans are more than mere pandering,” the Republican letter states. “They are a direct, immediate threat to the independence of the judiciary and the rights of all Americans.”

Whitehouse was joined in the brief by Sens. Mazie Hirono (Hawaii), Richard Blumenthal (Conn.), Richard J. Durbin (Ill.) and Kirsten Gillibrand (N.Y.). He said in an earlier interview that it was not Democrats who had politicized the court.

In 2016, Senate Republicans refused to hold a vote on Judge Merrick Garland, President Barack Obama’s Supreme Court nominee to fill the seat of Justice Antonin Scalia, who died that February. And when it appeared that Hillary Clinton would win the presidency and be in position to name the justice who could swing the court to the left, Sen. Ted Cruz (R-Tex.) floated the idea that a Republican Senate might continue to ignore the opening on the court and leave it at eight members.

Instead, when President Trump was elected, McConnell got rid of the filibuster to ease the confirmation of Neil M. Gorsuch to fill Scalia’s seat and Brett M. Kavanaugh to replace Justice Anthony M. Kennedy, who retired in 2018.

In the letter, Republicans say the number of justices will remain at nine: “While we remain members of this body, the Democrats’ threat to ‘restructure’ the court is an empty one.”