All 53 Republican senators joined together Thursday to call out their Democratic colleagues for what they described as a left-wing "assault" on judicial independence in reference to a recent brief they said amounted to a threat to pack the court.

In a letter to the Supreme Court, the senators warned about the danger posed by the Democrats who recently accused the conservative-majority court of being “not well” and suggested the public could demand it be “restructured” if it does not “heal itself.” This warning was part of a brief in which they argued that a case about a New York City gun law should be dismissed as moot.

SENATE DEMS DELIVER STUNNING WARNING TO SUPREME COURT: ‘HEAL’ OR FACE RESTRUCTURING

“But our colleagues did more than raise legal arguments in favor of mootness," the Senate Republicans said. "They openly threatened this court with political retribution if it failed to dismiss the petition as moot."

The brief -- filed Aug. 12 by Sen. Sheldon Whitehouse, D-R.I., and co-signed by Sens. Richard Blumenthal, D-Conn., Mazie Hirono, D-Hawaii, Richard Durbin, D-Ill., and Kirsten Gillibrand, D-N.Y. – claimed that the conservative-majority court “has employed a number of methods to circumvent justiciability limits in decisions that moved the law” and warned what could happen if they do not stop.

“The implication is as plain as day: Dismiss this case or we’ll pack the Court,” the Republicans said, claiming that the Democrats’ brief is an example of how “judicial independence is under assault.”

The letter referred to how members of Congress and 2020 Democratic candidates have supported the idea of expanding the number of justices on the Supreme Court, “in order to further their radical legislative agenda.”

Republicans warned that the Supreme Court “must not be cowed by the threats of opportunistic politicians.”

DEM SENATOR HIT WITH BAR COMPLAINT FOR ‘OPENLY THREATENING’ SUPREME COURT

Supreme Court Justice Ruth Bader Ginsburg came out against court-packing in a July NPR interview, saying, “Nine seems to be a good number.” The Republican letter says they “share Justice Ginsburg’s view[.]”

Whitehouse was defiant in the face of the Republican backlash.

“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," Whitehouse said in a statement. "They want us to shut up about their capture of the Court; we will not."

Senate Majority Leader Mitch McConnell, R-Ky., said in a statement that Democrats’ court-packing “threat” was “synonymous in American history with the idea of an unprincipled power grab.”

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After filing the brief, Whitehouse was slapped with a complaint filed with the Rhode Island state bar, accusing him of misconduct for “openly threatening” the court, and for allegedly practicing law without an active license. The complaint, filed by conservative watchdog group Judicial Watch, noted that Whitehouse’s Rhode Island law license was inactive, and that he is not licensed anywhere else.

A representative from Whitehouse's office told Fox News after the complaint was filed, "Senator Whitehouse is a member of the Supreme Court bar and filed nothing in Rhode Island," adding, "This attack group has no credibility."

Fox News’ Jason Donner contributed to this report.