Senate Democrats have not talked much publicly about what, if anything, they intend to do about filibustering — the practice whereby a minority of as few as 40 senators can block action — in the event that they secure a majority in November. And their line to the press is that they are focused on trying to win a majority before talking about how they’re going to use it.

But while speaking to a Philadelphia radio station about the importance of reelecting Pennsylvania Sen. Pat Toomey, John McCain explained exactly what Democrats are going to have to do — either eliminate or significantly curtail it.

“I promise you that we will be united against any Supreme Court nominee that Hillary Clinton, if she were president, would put up,” he said. “I promise you.”

His staff later clarified that he didn’t literally mean he would oppose them sight unseen out of blind partisanship, but rather that based on his knowledge of Clinton she will likely appoint justices who are too liberal for his taste.

This is an important statement from McCain because Senate Republicans’ stated reason for blocking the confirmation of Judge Merrick Garland to fill the Supreme Court vacancy is entirely different. What they are saying is that they are blocking him because it would be inappropriate to fill a vacancy in the eighth year of a presidential term.

What McCain is saying is very different: They are blocking him because he is a Democrat with ideological views that fit in line with the Democratic Party, whereas they are Republicans with different ideological views. This is in many ways a more sensible view — I wouldn’t want to vote for a Supreme Court nominee I disagreed with on important topics either — but it’s also one with more dramatic constitutional implications. If Democrats can’t count on even a few Republican votes for a Democratic nominee even in the wake of a big election win, then they have to change the rules and eliminate filibustering of Supreme Court nominees.

That, at least, is a dynamic that leads to a resolution. The real problem is if McCain gets his way and Republicans retain a senate majority. Tradition dictates that the Senate opposition can reject an ideologically extreme Supreme Court nominee but should offer a reasonable amount of deference to the president. That tradition has been under pressure in the past four Supreme Court confirmation fights, and McCain is saying it’s over — Republicans should offer no deference to Clinton whatsoever.

Democratic partisans are professing outrage at this, but it’s hard not to sympathize with the GOP. As a US senator, Clinton herself opposed both of Bush’s Supreme Court nominees — why should a senator vote to confirm a justice whose views she doesn’t approve of?

The problem is that the system as a whole relies on that kind of deference to prevent the endless proliferation of judicial vacancies. McCain’s principle would produce a kind of mini-crisis in constitutional government, and the 4-4 split on the Supreme Court would prevent the resolution of other constitutional disputes. This kind of irresolvable conflict is, unfortunately, baked into America’s system of government — a system that really only works if the parties aren’t ideologically disciplined and polarized.