But over time, both parties — but particularly Republicans at the start of Bill Clinton’s presidency and then again at the start of Barack Obama’s presidency — began using the filibuster against more and more things, eventually winding up in 2009 with a Senate in which in effect everything was filibustered — everyone understood that (with the exception of a few things that were exempt) there was no chance anything could pass with just a simple majority. It had not always been like that; as recently as 1991, the highly controversial Supreme Court nomination of Clarence Thomas was confirmed by a 52-48 vote.

The more filibusters, the less stable the filibuster rule became. Democrats never did come close to changing the rule during Bill Clinton’s presidency, probably because the administration didn’t really get organized until after it had squandered its two years of unified Democratic government. Republicans, however, threatened to eliminate the filibuster for judicial nominations when Democrats used it against a handful of George W. Bush’s judicial nominees they considered out of the mainstream. They backed off only when seven Democrats and seven Republicans cut a deal that Republicans would not impose filibuster reform as long as, in effect, Democrats stopped using it. That probably wasn’t a stable compromise, but soon after Democrats took the Senate majority, so it no longer mattered.

Then during Barack Obama’s administration Republicans didn’t just target specific nominees — they used the filibuster against positions, not people, blockading vacancies on the District of Columbia Circuit Court and in several executive branch agencies. That, too, was unstable, and when Republicans refused to cut a deal, Democrats had little choice but to change the rules by majority vote in 2013.

Mr. McConnell says that Democrats lived to regret that change, but it’s a preposterous claim. Republicans broke all precedents and norms when they regained the majority in the 2014 elections by refusing to consider any Supreme Court nominee from Barack Obama and by largely shutting down confirmation of appellate court and many executive branch positions as well. They then removed the filibuster for Supreme Court nominations as soon as Democrats tried to use it, and then eliminated the “blue slip” procedure that Democrats had retained when Obama was president, which allowed senators an effective veto over judicial nominations in their own state.

It’s impossible to prove what would have happened had Democrats not acted in 2013, but there’s just no reason to believe that Republicans, who at least since 2009 have had zero respect for confirmation norms, would have acted any differently. Which means that confirmations of Mr. Obama’s nominations would have ended in 2013, rather than in 2015, and Republicans would have filled all of those additional vacancies after 2016.