The fight over statehood shouldn’t be about partisan interests but the durability of the Constitution in a time of deepened polarization. As David Rivkin and Lee Casey noted in these pages in July, the Founders created a federally controlled district in the seat of U.S. government to maintain federal sovereignty. If D.C. were a state, it could use its power as leverage over the national government by withholding public services or not providing security.

As for Puerto Rico, the U.S. policy since the 1950s has been to move the territory toward statehood if it wishes. Yet strong Puerto Rican resistance—as well as a significant population that wants an independent commonwealth—remains. The results of a 2012 referendum are disputed, and a subsequent referendum was boycotted by the anti-statehood side. By admitting the territory in an act of partisan brinkmanship, a polarized U.S. would absorb a state with a secessionist movement.

That’s not to say new U.S. states should be ruled out indefinitely. But making Senate-packing a new front in America’s cycle of partisan escalation could delegitimize institutions in ways even advocates can’t imagine. Mr. Trump has undermined many 20th-century American political norms. If Democrats get power, they need to decide if they want to restore normalcy or act on their own version of 19th-century scorched-earth politics. We wish we could say the latter outcome isn’t more likely.