At the New Republic, from Jeff Sheshol, one of the leading scholars of the political battle over FDR’s Court-packing plan:

When Roosevelt finally struck against the court, at the start of his second term, his party held 76 of 96 seats in the Senate; he saw no reason why he could not persuade Congress to expand the court and, at the same time, renew his assault on the economic crisis. Yet his court plan cost him his mandate. It consumed Congress for nearly six months, dividing his supporters and driving some into the arms of the GOP. In May 1937—nearly four months into this war of attrition—Roosevelt proposed one of the most sweeping pieces of legislation of his presidency: a bill to ban child labor in factories and to create a labor standards board that would regulate wages and hours. He did this, in part, to change the subject. The gambit failed. The bill did nothing to draw recalcitrant Southern Democrats back into the fold. “To ask them simultaneously to support a national wages-and-hours law” and the court bill, this magazine observed, “is probably to ask more than human flesh is capable of.” 

The New Deal never regained its momentum. While the court plan did not create the divisions in the Democratic Party, it wrenched them wide open; it left the party raw and at war with itself. . . .

History, therefore, as well as political logic and math, counsel Biden against moving rashly. Even the more bullish blue-wave projections would give him nothing approaching FDR’s 60-vote margin in the Senate. At best, Biden might have three or four votes to spare. To advance his agenda—really, to do much of anything at all—he needs to avoid alienating Democratic senators like Joe Manchin of West Virginia with a costly fight over the court. He will also need the votes of senators who have just recaptured Republican seats (most likely in Arizona, Colorado, and Maine), as well as those who will face reelection in 2022 in states that are less-than-reliably blue: New Hampshire, for example, and Nevada. 

Of course, Biden’s calculus could change over time. The party’s left wing—as Senator Ed Markey has made clear—is determined to rebalance the court. And if oral arguments begin to look, increasingly, like meetings of a student chapter of the Federalist Society (“Government Overreach in the Wake of COVID-19” was the title of one such session in September at Baylor University), the court will find itself at odds not only with a Biden administration but with “the plainest facts of our national life,” as Chief Justice Charles Evans Hughes put it in 1937. Every time a bad decision comes down—a decision restricting abortion rights, for example, or discounting the effects of racial discrimination, or blessing the purchase of political influence by business interests, or constraining the government’s ability to address the climate crisis—the calls to pack the court will grow more insistent. We are no longer terrified of the phrase, but perhaps the justices in the majority should be.