It’s starting to look like Senate Majority Leader Mitch McConnell and the Republican leadership would be wise to move quickly to replace Senator Grassley as chairman of the Judiciary Committee. That’s because the Iowan has indicated he won’t hear a nominee to the Supreme Court if President Trump sends the Senate one in 2020, the last year of his current term.

That news came in an interview with Martha MacCallum of Fox. “If I’m chairman, they won’t take it up,” Mr. Grassley allowed in the rambling way in which he speaks. “No, because I pledged that in 2016, that if the ball’s the same as it is. Now, if somebody else is the chairman of the committee, they’ll have to decide for themselves. But that’s a decision I made a long time ago.”

Mr. Grassley is trapped in the corner into which he painted himself after President Obama nominated Judge Merrick Garland to the seat previously held by Justice Antonin Scalia. Judge Garland is a fine sage, but he was not for what the Senate’s Republican majority was looking. They declined to give him a hearing. Had they heard his views on the 2nd Amendment, he wouldn’t have cleared the Senate.

There is, though, a big difference between the circumstances of 2016, when Judge Garland was nominated, and the situation that will obtain if Mr. Grassley is still Judiciary chairman in 2020. In 2016, the president was, in Mr. Obama, a Democrat and the Senate was controlled by the Republicans. With balance of the court so close, the incentive is for each party to play hardball.

It’s a completely different kettle of judges, though, if the the Senate is controlled by the same party as the president. Which is what would be the situation if, come 2020, Mr. Grassley is still chairing Judiciary. That would mean that the Republicans still held a majority in the upper chamber. It would also mean the Senate would get a nominee more to its liking.

Why would a Republican-majority shrink from hearing any Trump nominee? They all ran on the same platform language in respect of the Supreme Court. It’s not as if there’s a constitutional rule mandating what the Senate has to do in this situation. The only constitutional language is that each house of Congress “may determine the rules of its proceedings.”

Leader McConnell seems to get this. As well as who started the collapse of civility in confirmations. The infamous depredations of Supreme Court nominees Bork and Thomas were the handiwork of Democrats. It was in the 108th Congress that they started systematically filibustering nominees to the appeals circuits, including such luminaries as Miguel Estrada and Janice Rogers Brown.

Talk of a “nucular” option — doing away with the filibuster — came to a head in the 109th Senate, when the GOP had a 55 to 45 seat advantage and were still worried about getting cloture on a circuit nominee, Priscilla Owens. That’s how the Gang of 14 emerged. The idea was to avoid the Republicans using the nuclular option. Justice Alito got onto the bench because of the 14.

So, by the way, did a young circuit judge named Brett Kavanaugh, though by then the Gang of 14 had begun to fray. The pact went by the wayside with the 110th Senate, when the Democrats gained a slim majority. Come 2013, Majority Leader Harry Reid did away with the filibuster for nominees other than to the Supreme Court. Once Mr. McConnell became majority leader, he carried things to the next step.

Our own expectation is that the voters are going to sort this out. Are they really worried about the Senate rules? Our guess is that are going to get down to the substance. What kind of court do they want? If the Senate doesn’t give it to them, the voters will take things into their own capacious hands. No one can say whether there will be a court opening in 2020. Best to move now on Mr. Grassley, though, so as to be prepared if it happens.