There is nothing in the Constitution mandating that the Supreme Court have nine members, and a simple act of Congress could increase that number to 11, or 15, or even more. That effectively creates a way for a political party in control of the House, Senate, and presidency to add a large number of ideologically sympathetic justices to the Court, all at once.
To many leftists and left-liberals, such drastic action is needed if any progressive legislation in the future is to survive. The concerns in question have less to do with hot-button social issues like abortion and LGBT rights and more to do with the constitutionality of economic regulation and redistributive programs. Just this past Wednesday, the Supreme Court effectively gutted public sector unions under the guise of the First Amendment. In 2012, there were four votes on the Supreme Court (including Anthony Kennedy) for striking down the Affordable Care Act in full. And there’s an emerging movement of judicial conservatives, championed prominently by Donald Trump’s appellate appointee Don Willett, which wants the courts to become much more aggressive in blocking economic regulation.
If that kind of judicial conservatism comes to dominate the Supreme Court, then even winning back the White House and Congress won’t be enough for programs like a $15 minimum wage, or Medicare-for-all, or a free college plan, to be passed and secured. The Supreme Court would stand ready to rule them unconstitutional nearly as soon as they are passed. In such a scenario, court-packing starts to look like a reasonable defensive measure.
The prospect of a Court slapping down progressive economic measures brings to mind the last time court-packing was seriously considered. In 1937, Franklin Roosevelt was facing off with a hostile Supreme Court that routinely ruled aspects of the New Deal unconstitutional on the same grounds. During that era, the Court interpreted the due process clauses of the 5th and 14th Amendments as sharply limiting economic regulation and ruling out things like federal bans on child labor, minimum wage laws, and legislation limiting workweeks to 60(!) hours.
Roosevelt’s plan to increase the Court’s size — which would have allowed him as many as six new justices, for a 9-6 majority for the New Deal on a 15-member court — ultimately failed in the Senate, but not before successfully pressuring Justice Owen Roberts to switch his alignment from the Court’s conservatives to the liberals and rule for the constitutionality of minimum wage laws and the National Labor Relations Act.
If calls for court-packing grow loud enough, you could see something similar happen on the current Court, if some of the Republican-appointed justices like John Roberts or Brett Kavanaugh start moderating their decisions to prevent the radical disruption of the Court by Democrats who fear being locked out of policy influence for a generation.