The Democrats' brief urged the Supreme Court not to take up a challenge to New York City's gun laws, writing that the courts "do not undertake political 'projects.' Or at least they should not."

Progressives praised the brief and viewed it as a warning shot to the court to change tactics or face growing calls for structural reforms. Brian Fallon, the executive director of Demand Justice — a progressive group that has floated expanding the number of Supreme Court justices — called the brief a "badass move." ThinkProgress, which is affiliated with the left-leaning Center for American Progress Action Fund, characterized the Democratic senators as declaring "all-out war" on the Supreme Court.

Senate Republicans, in their letter, argued that the brief was the latest example of the independence of the courts being "under assault."

The courts have emerged as a lightening rod issue during the Trump era. Progressive groups have tried to make structural reforms like expanding the Supreme Court a litmus test for 2020 Democratic presidential candidates.

"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," Republicans wrote in their letter."For our part, we promise this: While we remain Members of this body, the Democrats’ threat to 'restructure' the Court is an empty one. We share Justice [Ruth Bader] Ginsburg’s view that 'nine seems to be a good number.' And it will remain that way as long as we are here," they added.