My colleague Todd Henderson has an opinion piece at Newsweek arguing that court-packing—adding additional Justices to the Supreme Court, for the purpose of changing the Court's decisions—is unconstitutional. I don't agree with the piece, but it has already attracted a ton of criticism and that criticism deserves more scrutiny.
First, basic background. During the New Deal, Franklin Roosevelt threatened to pack the Supreme Court. But in the end he didn't. There is a scholarly debate about whether the Court changed course in response to the threat, and also about whether President Roosevelt would have prevailed if the Court had acted differently.
One remarkable document that emerged from that conflict is the report from the 1937 Senate Judiciary Committee, on which Todd relies, which argued at length that court-packing for the purpose of manipulating the Supreme Court was unconstitutional, because it violated the spirit of the Constitution and the separation of powers. Before you dismiss challenges to court-packing as frivolous, you should really read it. And these arguments appeared much more widely in the legislative debate at the time as well.
Read more at The Volokh Conspiracy