“The Senate has proved its ability to reach bipartisan solutions to some of the most pressing challenges facing our nation,” Senator Mitch McConnell wrote last week not on the satirical news site The Onion, but for Fox News.

“After years of rhetoric,” the irony-free op-ed said of Mr. McConnell’s Democratic adversaries, “it’s hardly news that some are more interested in fanning the flames of division than reaching across the aisle.”

At a news conference the day after the midterm elections, the Senate majority leader warned that it would be “presidential harassment” if the newly elected Democratic majority in the House used its constitutional prerogative to act as a check on a president who has pushed ethics, decency and the rule of law to the brink. It was as if Mr. McConnell’s obstructionism in the Obama years had never happened.

But after two years of pushing for extreme legislation and appointments that succeeded or failed on party-line votes, Mr. McConnell is now putting the brakes on what is likely one of the only true examples of bipartisanship of the Trump era so far: a criminal justice reform bill that has brought together the American Civil Liberties Union and the Fraternal Order of Police, Republican conservatives like Senator Charles Grassley of Iowa and Democratic liberals like Senator Richard Durbin of Illinois.