And he published a USA Today op-ed this week writing that "so-called 'criminal-justice reform' ... is just a misguided effort to let serious felons out of prison."

A White House official said this week that Cotton had been an "ally" to the president on other issues but they believed they could get the votes needed to pass the bill in the Senate without him.

But his entrenched opposition underscores the uphill battle the legislation faces in getting scheduled for a vote in the Senate, where lawmakers are running out of time and GOP leadership has been noncommittal about whether they will bring it up before the end of the year.

Then, leadership would weigh the legislation against other issues that need to clear Congress in the lame duck session.

John Kennedy John Neely KennedyMORE In addition to Cotton, Sen.(R-La.) said Thursday that he will object if leadership tries to skip over procedural hurdles in scheduling a vote.

If Kennedy follows through, that would force McConnell to decide between eating up days of floor time during a tight schedule or kicking the issue to next year.

Grassley appeared to take aim at Whitaker on Friday, saying in a tweet that he expects the Justice Department to support his legislation.

"Now w Sessions out as AG + Pres Trump endorsing crim justice reform I expect DOJ to support the bipartisan/tough on crime/fair First Step Act especially since Acting AG Whitaker told me he would support bill if Trump did," Grassley said.