WASHINGTON — I was wrong about Mitch McConnell.

I doubted he would ever put a criminal justice measure that divided Republicans and ran counter to his conservative political instincts on the floor for a vote. But Mr. McConnell, Republican of Kentucky and the Senate majority leader, has, after years of delay, finally agreed to do so. On Thursday, he set in motion a Senate debate on the popular legislation seen as an overdue corrective to overly harsh sentences enacted in the tough-on-crime 1980s and ’90s.

Here is what it took: vocal support from President Trump, who is more popular than Mr. McConnell with Republicans and is hungry for a bipartisan victory. Demands for action from Senator Charles E. Grassley, the Judiciary Committee chairman who has been relentless on Mr. McConnell’s behalf when it comes to judicial confirmations and who wants to see the bill become law before surrendering the chairmanship next month. Verifiable support from well over a majority of Mr. McConnell’s Republican colleagues. And pressure from Paul D. Ryan, the departing House speaker who has been a cooperative partner of Mr. McConnell’s and also wants the bill passed on his watch.

That’s not to mention that Mr. McConnell sought guarantees that Democrats would embrace the measure. He faced strong calls for passage from Jared Kushner, the president’s son-in-law and senior adviser who has taken on criminal justice overhaul as a personal crusade and a way to demonstrate domestic policy expertise. Strong backing also came from the billionaire Koch brothers, the conservative powers who have helped bankroll Mr. McConnell’s majority. Other conservative groups that have been distrustful of Mr. McConnell in the past also supported the proposal and urged him to put it to a vote. Finally, endorsements from Senate Republicans who had previously opposed the bill seemed to be the clincher.

[Read about some of the real people who lobbied for the criminal justice bill before Congress.]

Compared to those McConnell Rules, the Hastert Rule looks downright flexible. Mr. McConnell didn’t just require backing from a majority of his majority, as the Hastert Rule suggests; he wanted every conceivable guarantee that the criminal justice measure would not blow up on him politically. And it still required a tremendous amount of pressure from all corners to persuade him to do it.