Once the two Democratic winners of Georgia’s Senate runoff races are sworn in, around January 20, the two parties will each hold 50 seats in the Senate. On the face of it, that means the Democrats will control the chamber, since the Constitution gives the vice president a vote on the floor when the Senate is equally divided. But as a practical matter, it may well mean something more like shared control of the Senate in some key respects, and exactly what that will look like will need to be worked out in short order.

The Democrats can’t really count on …