As President Donald Trump and Senate Majority Leader Mitch McConnell move to confirm Amy Coney Barrett on the Supreme Court, progressive Democrats are increasingly talking about packing the Supreme Court in retaliation — increasing the size of the court and adding new, liberal justices to the bench.

At a minimum, the idea rests on the heroic assumption that Democrats will win the presidency and both houses of Congress. But that's not all. History demonstrates the grave difficulty of successfully mustering even majority-party support to add new Supreme Court seats.

To pack the court, Democrats would need more than just the prospect of a conservative court. They would need a sustained argument that the court had become fundamentally illegitimate by its composition or its conduct.

The archetypal example of attempted court-packing came in President Franklin Delano Roosevelt's second term in office.

Democratic frustration with the court was at a high point. The conservative, libertarian-leaning majority of the court had been striking down progressive wage and hour regulation for three decades during what came to be known as the Lochner era (named for the 1905 case of Lochner v. New York, which struck down a 60-hour workweek for bakers). Then, in 1935, the court struck down the two cornerstones of Roosevelt's New Deal, the National Industrial Recovery Act and the Agricultural Adjustment Act.

Roosevelt got no Supreme Court appointments in his first term. But in the 1936 election, he won re-election with more than 60% of the vote and carried the Electoral College 523-8 — a landslide by any measure. The Democrats held a stunning 74 seats in the Senate to the Republicans' 17, and ruled the House of Representatives by a margin of 334-88.

Thwarted by the Supreme Court, and frustrated by the thwarting, Roosevelt in 1937 proposed legislation that would have added a new justice for each one over the age of 70. Six justices were over 70, so the law would have let Roosevelt transform the court immediately.

The Republicans had no chance of stopping the court-packing plan. Yet remarkably, many Democrats objected, suggesting that Roosevelt was trying to change the balance of power and make himself a dictator. With Adolf Hitler having come to power in Germany, the charge resonated.

The justices themselves — including the court's liberals — also disapproved. Roosevelt had argued that the old justices couldn't keep up with their work. In a letter to the Senate Judiciary Committee, Chief Justice Charles Evan Hughes, liberal lion Justice Louis Brandeis and conservative stalwart Justice Willis Van Devanter denied the charge.

The crucial events in the plan's short life came in March and April of 1937. First the court, in a 5-4 opinion written by Hughes, repudiated the Lochner doctrine and upheld wage and hour legislation passed by states. The decision was interpreted by the public as a result of the pressure imposed by the court-packing effort. That took the wind out of the plan's sails. The decision came to be known as the "switch in time that saved nine."

Then the court upheld the Wagner Act, a signature piece of New Deal legislation that created the National Labor Relations Board. The message was that the court would uphold the second New Deal.

Van Devanter soon announced his retirement, further weakening the logic of court-packing. When, in July, Sen. Joe Robinson, who had been promised a Supreme Court seat in exchange for leading the court-packing plan, died in his sleep, the scheme died with him.

The historical lesson is, arguably, that even with near-total partisan power and good cause, FDR couldn't pack the court. If the lesson is still valid, it suggests that it would fail again now.

Even a threat to overturn Roe v. Wade might not be enough to get Democrats to pack the court. The effects of reversing abortion rights would be dire for poor women in red states, who would face severe difficulties traveling to blue states to terminate unwanted pregnancies. Yet it is already hard for women in red states to get abortions. It is far from clear that increased difficulty would suffice to get Democrats to eliminate a check on the power of a president who also controls Congress.

And many Democrats would worry about removing the court's check on future Republican presidents.

No doubt, pro-packing partisans would point out that Trump was elected in 2016 without a popular majority (as was George W. Bush in 2000), and that the non-majoritarian design of the Senate means that a minority of Americans have put the Senate in Republican hands. Combined, these features of our non-majoritarian constitutional design could give us a Supreme Court that does not at all reflect the will of the majority. That in turn could delegitimize the court and provide a basis for packing it.

The trouble with this argument is that, to liberals, the appeal of the court is precisely that is a counter-majoritarian institution. The emergence of judicially upheld gay rights are a perfect example of how the court protected minority rights against what were once majority attitudes.

The court's recent decision extending workplace anti-discrimination law to LGBTQ people is the latest instance of the court doing what Congress was not prepared to do via new legislation. In this sense, the decision wasn't majoritarian, even if the court's majority insisted it was only enforcing the plain meaning of the 1964 Civil Rights Act. Liberals loved it anyway.

Court-packing is certainly conceivable if, for example, the court were to give the presidency to Trump after a contested election. That could lead Democrats to a four-year condemnation of the court and a change in attitudes deep enough to demand a change in institutional design. Short of that kind of outrage, however, it seems more likely that energy for court-packing would diffuse in the case of a Democratic victory in November.

If FDR couldn't do it, can Joe Biden?

Noah Feldman is a professor of law at Harvard University and was a clerk to U.S. Supreme Court Justice David Souter. He is host of the podcast "Deep Background" and his books include "The Three Lives of James Madison: Genius, Partisan, President."