“The Supreme Court is not well. And the people know it,” writes Whitehouse, who is listed as the attorney of record on the friend-of-the-court 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.

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Democratic Sens. Mazie Hirono (Hawaii), Richard Blumenthal (Conn.) Richard J. Durbin (Ill.) and Kirsten Gillibrand (N.Y.) joined the incendiary brief, which questions whether the court’s conservative majority — nominated by three Republican presidents — is motivated by partisan intent and is in the pocket of the National Rifle Association and the Federalist Society, a conservative legal group.

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“Out in the real world, Americans are murdered each day with firearms in classrooms or movie theaters or churches or city streets, and a generation of preschoolers is being trained in active-shooter survival drills,” Whitehouse writes. “In the cloistered confines of this Court, and notwithstanding the public imperatives of these massacres, the NRA and its allies brashly presume, in word and deed, that they have a friendly audience for their ‘project.’ ”

The brief has lit up the right. Senate Judiciary Committee Chairman Lindsey O. Graham (R-S.C.) called it an extraordinary threat from one branch of government to another and tweeted: “Packing the Supreme Court . . . Bad idea. Liberal dream. Trump’s 3rd term is looking better and better.”

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The Wall Street Journal editorial board dubbed it the opposite of an amicus filing — an “enemy-of-the-court brief” — and the National Review’s David French called it “astonishing.”

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“It is easily the most malicious Supreme Court brief I’ve ever seen,” he wrote. “And it comes not from an angry or unhinged private citizen, but from five Democratic members of the United States Senate.”

In an interview, Whitehouse was unapologetic, saying he was cautioning the court, not threatening it.

“In the same way that you might warn somebody walking out on thin ice — ‘Hey, the ice is thin out there, you want to be careful, maybe you want to come in’ — I think that was the motivation for filing this brief,” said the former U.S. attorney and state attorney general.

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“To warn the court that it already has its reputation in some degree of trouble . . . it’s getting to the danger that they might fall through the ice.”

The Supreme Court in January said it would hear New York State Rifle & Pistol Association v. City of New York, a case involving some unique-to-New-York restrictions on how gun owners with 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.

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It is the first Second Amendment case the court has accepted in a decade, and it came after the NRA-endorsed Justice Brett M. Kavanaugh replaced the more moderate Justice Anthony M. Kennedy on the closely divided court. Gun-control advocates worried that the case would provide a chance for the new majority to establish a right to carry a weapon outside the home, or impose heightened judicial scrutiny on gun control laws.

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The city of New York rescinded the regulations that the gun groups and owners had objected to, and the state legislature passed a law prohibiting their reinstatement. The two sides are now sparring over whether that renders the case moot, something the court is scheduled to consider Oct. 1.

Whitehouse’s brief is one of about three dozen filed with the court, and it is ostensibly on that issue of mootness.

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Federal courts are restricted to deciding actual “cases and controversies” brought by plaintiffs who suffer real harms. Whitehouse said New York’s actions have given the plaintiffs all they seek, and the court should not become a “partner in a ‘project’ to expand the Second Amendment and thwart gun-safety regulations.”

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But from there, Whitehouse’s brief is more of a compendium of complaints that he and other Democrats have made about the court. He denounces the NRA’s endorsement of Kavanaugh, the role of Federalist Society officials in promoting judicial candidates, and the “dark-money” campaigns to promote Kavanaugh and President Trump’s other Supreme Court nominee, Justice Neil M. Gorsuch.

It accuses the court’s conservative majority of toeing the “corporate and Republican” line in “areas like voting rights, partisan gerrymandering, dark money, union power, regulation of pollution, corporate liability, and access to federal court, particularly regarding civil rights and discrimination in the workplace.”

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The brief won praise from the left.

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“This filing is a bada-- move by these Senate Democrats,” said Brian Fallon, executive director of the liberal group Demand Justice. “The Republican justices on the Supreme Court should be on notice that the public is quickly losing faith in the court as a legitimate institution.”

Those who have criticized Whitehouse’s brief say it is no different from Trump’s complaints that the judges who rule against his initiatives are partisans. They contend the brief is silent about groups on the left whose funding is veiled. Liberal politicians never note that the court’s four Democratic-nominated justices vote as a bloc in many of the court’s most sensitive cases, they say.

But the brief is acknowledgment of the partisan battle over the federal judiciary.

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“Extremely concerning to see Senate Democrats threaten federal judges like this,” Sen. Ted Cruz (R-Tex.) tweeted.

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But in 2016, Senate Republicans refused to hold a vote on Judge Merrick Garland, President Barack Obama’s Supreme Court nominee. And when it appeared Hillary Clinton would win the presidency and be in position to name the justice that could swing the court to the left, Cruz floated the idea that a Republican Senate might continue to ignore the opening on the court and leave it at eight members.

In the interview, Whitehouse said his “fact-finding” brief assembled the equivalent of an indictment of the court. “I think there’s a point at which somebody has to say the emperor has no clothes,” he said

He said he did not know whether the justices would find it persuasive. The Wall Street Journal hoped it would have the opposite effect.