For months, Mr. McConnell had assured reformers that, if a bill emerged with the support of at least 60 members, he would bring it up for a vote. But no sooner had Mr. Trump finished praising the First Step Act than Mr. McConnell began weaseling out of his pledge, clucking about all the pressing business the Senate faced and how there most likely wouldn’t be time to tackle this issue after all.

By now, there’s little point in criticizing Mr. McConnell for jamming things up. Obstructionism is what defines him — and is, in fact, among the “ achievements ” in which he has professed the greatest pride. More particularly, criminal justice reform is not a core concern of the Republican base. And with hard-liners such as Senator Tom Cotton of Arkansas loudly trashing the bill, allowing a vote could lead to an awkward moment or two for Mr. McConnell’s conference. This the majority leader cannot abide, no matter how worthy the cause or how broad its support.

Now is when Mr. Trump’s much ballyhooed salesmanship skills could really come in handy. “If we get to it this year, it’ll be largely because of White House pressure,” Senator Roy Blunt of Missouri, a member of the Republican leadership, told CNN last week .

The president isn’t exactly shy about using his bully pulpit, and he has shown himself happy to pillory lawmakers, Mr. McConnell among them, when displeased with Congress’s lack of progress. But confronted with Mr. McConnell’s neglect of criminal justice reform, Mr. Trump has remained uncharacteristically passive.

He has reportedly phoned the Senate leader to ask him to take up the bill. But those involved with the reform push say they’ve seen little sign that Mr. Trump is exerting himself or willing to burn any political capital on this issue. The president has been unable even to rouse himself for a Twitter tirade, issuing only a single, cheery tweet on the topic in the weeks since his endorsement.