The president handed conservatives an issue to paint him as a sort of American dictator. “The whole New Deal really went up in smoke as a result of the Supreme Court fight,” said Secretary of Agriculture and future Vice President Henry Wallace. The plan became a key talking point to foment a fierce political backlash against the F.D.R. and the liberal wing of the Democratic Party. One constituent sent a senator a bullet wrapped in a paper that warned, “If you support Roosevelt’s court bill, we will get you — you dirty rubber stamp.”

The court battle was a huge reversal of fortune for Roosevelt. Starting with the 1938 midterm elections, a coalition of Southern Democratic committee chairmen and Republicans came to power. They broke with the president by forming a powerful congressional voting bloc that had the muscle to stifle major initiatives at the high point of the liberal era.

Health care and civil rights legislation, for instance, were two issues that the coalition held hostage. Southern committee chairmen worked with ranking Republicans to prevent legislation from ever reaching the floor. Senator James Eastland of Mississippi joked about special pockets that he had put into his pants to carry around the civil rights bills that he never allowed to come up for a vote. When bills did reach the Senate, the coalition joined forces to filibuster. The columnist William White called the Senate the “South’s unending revenge upon the North for Gettysburg.”

The only real break for liberals finally came in 1964 and 1965, when an enormous grass-roots civil rights movement and huge Democratic majorities put into office during the 1964 election gave President Lyndon Johnson a brief window to pass Great Society legislation — a window that largely closed with the midterms in 1966. Ultimately, the conservative coalition that was born out of Roosevelt’s court-packing battles came apart only when congressional reforms in the mid-1970s brought the committee era to an end.

If Democrats place their bets on court-packing once again, the political backlash would probably be even more severe. The right now possesses huge institutional weapons — national news media outlets, well-established interest groups and think tanks, and vast sources of campaign money — that could quickly be deployed. They have invested heavily in reshaping the courts over the past few decades. Given that the Republicans have tended to care about the courts more than liberals, it is likely that court packing could be more effective at energizing Republicans than Democrats. Even if there is a Democratic Congress before 2020, proponents will never find the two-thirds support needed to overturn a presidential veto.

If liberals want to change the direction of the courts, they should do more to replicate the kind of long-term projects their opponents have undertaken since the 1980s to nurture judicial talent and create a deep pool for future appointments. Liberals need to do better at using congressional procedure and handling open hearings to diminish political support for conservative nominees — and obviously they need to win elections, starting in November.