Other academics agree that Democratic court packing is likely. "If the GOP goes forward with trying to fill the seat this year regardless of the election result, I think there is a substantial likelihood (at least 50% or more) that the Democrats will respond with court-packing, the next time they get a chance to do so," George Mason University law professor Ilya Somin said via e-mail.

But court packing would be a "terrible idea," Somin warned, "as it would lead to a spiral that undermines the institution of judicial review." It would also open the door for Republicans to respond in kind. "I don't see any likely way of avoiding the spiral, once one party has passed a court-packing bill once," he said.

Expanding the court isn't the only way to respond to what Democrats would perceive as an illegitimate election-year appointment, however. In a paper authored this year by Yale Law School professor Samuel Moyn and University of Chicago law professor Ryan Doerfler, the two suggest several options to overhaul the court by reducing its power.

The authors said lawmakers could require a Supreme Court supermajority for some decisions, so that major federal statutes aren't invalidated by a simple 5-4 ruling. Congress could also insulate specific legislation from judicial review — known in legal circles as "jurisdiction stripping."

These changes "would shift important policy disputes from the judicial arena to the small-d democratic one, thereby substantially reducing the importance of which party has effective control of the courts," Doerfler told NPR.