NO ONE should be happy about what happened in the Senate on Thursday. We do not just mean that, as many senators have recognized, both sides will at some point regret eliminating the filibuster on Supreme Court nominees. Senators of both parties should also take no pride in the role they played in moving their institution to this point. Both sides claim with a straight face that they were forced into this descent by the relentless and one-sided unreason of their opponents. The claim does not stand up for either party.

Now the question will be whether senators can, against all trends, find a way to avoid doing yet more damage.

Democrats, now claiming to be the injured party, unreasonably obstructed well-qualified judicial appointments of President George W. Bush. The judicial filibuster survived then only because a bipartisan group of 14 senators struck a deal in 2005 that allowed for some nominees to be approved but preserved the minority’s right to demand 60 votes for appointees they particularly disliked.

The deal did not lower tensions for long. Republicans turned up the dial on obstruction during President Barack Obama’s years, halting even moderate appeals court nominees. The Democrats responded in 2013 with the “nuclear option,” eliminating the filibuster on all non-Supreme Court judicial nominees to push Mr. Obama’s picks through by majority vote. Though the GOP obstruction was deeply unfair, Democrats were deeply mistaken to change the Senate in this unilateral way.

After they regained control of the Senate in 2014, Republicans refused to give Judge Merrick Garland even a hearing after Mr. Obama nominated him to replace Antonin Scalia on the Supreme Court. Democrats retaliated this week by filibustering President Trump’s pick for that seat, Neil Gorsuch. Another bad call; but it’s a bit rich to hear Majority Leader Mitch McConnell (R-Ky.) bemoan the Gorsuch filibuster as the first ever when he didn’t allow Mr. Garland a vote or anything else. On Thursday, Republicans retaliated in turn, deploying their own nuclear option and eliminating the filibuster on Supreme Court appointees, too, in order to push Mr. Gorsuch through. Another fateful mistake: From here, the judicial appointment process will become even more partisan.

“This is precisely the outcome our Founders feared, when lifetime appointments to our highest court, which touches every aspect of American life, become just another partisan exercise,” Sen. Michael F. Bennet (D-Colo.) said before the vote. “If you don’t like the ring of Justice Gorsuch, how do you feel about Justice Pruitt (who doesn’t believe in climate change); or Justice Sessions (who has a record of opposing civil rights and equality). If we continue down this path, both could be confirmed with a slim majority vote.” That is the path the Senate chose Thursday.

Mr. McConnell insists that eliminating the filibuster on judicial nominees does not mean the Senate will eventually ditch the 60-vote threshold on legislation, too. Well, best of luck. Though lately used with unwarranted frequency, the ability to filibuster bills tends to force those in power to moderate their goals and seek policies that can attract at least some buy-in from the other side. In an era of heightening partisanship, that moderating force is all the more important.