The principal argument against Democrats’ expanding the Court is that Republicans will simply respond in kind the next time they have the opportunity to do so, inciting a never-ending retaliatory spiral. However, there are three powerful responses to this argument.

First, Republicans initiated this arms race by stealing the Court vacancy that was rightfully Obama’s to fill in 2016. For Democrats to refrain from responding in kind would be to disarm unilaterally. As game theorists have shown, the quickest way to restore a stable equilibrium after one party in a reiterative game has repudiated a cooperative norm is to retaliate in kind. Failing to do so incentivizes only more of the norm-breaking behavior.

Second, Republicans themselves will almost surely enlarge the size of the Court the first time they have the opportunity and see the need to do so. Suppose that Democrats refrain from expanding the Court in 2021–22. Next, imagine that two Republican justices are tragically killed in an automobile accident, and President Biden and the Democratic Senate replace them with two Democratic justices, restoring Democrats to the five-to-four majority they ought to have enjoyed after Scalia’s death. Now suppose that, in 2024, a Republican is elected president and Republicans win control of Congress.

Does anyone doubt that Senate Majority Leader McConnell would pursue Court expansion?