WASHINGTON ― The U.S. Senate will vote on a bipartisan bill that would reduce some prison sentences, Senate Majority Leader Mitch McConnell (R-Ky.) said Tuesday.

In remarks on the Senate floor, McConnell said he would add the First Step Act to the Senate’s busy schedule, which already includes a must-pass government funding bill and farm legislation that has taken months to hash out.

President Donald Trump previously endorsed the bill and the White House has been lobbying the Senate to hold a vote.

The surprise announcement follows weeks of recalcitrance from McConnell to move forward on the matter. Last week, he informed White House officials that a vote on the bill was “unlikely” before the end of the year.

Passing the criminal justice legislation before the end of December will require significant cooperation. Any one senator has the right to object and delay the several procedural hurdles necessary to get the bill to the president’s desk. The task may take as little as three days or as long as a week, depending on whether opponents of the bill like Sen. Tom Cotton (R-Ark.) choose to gum up the works.

“Unless we approach all this work in a highly collaborative, productive way and take real advantage of unanimous consent to expedite proceedings, it is virtually certain that the Senate will need to be in session between Christmas and New Year’s in order to complete this work,” McConnell said Tuesday.

BRENDAN SMIALOWSKI via Getty Images Senate Majority Leader Mitch McConnell (R-Ky.) said Tuesday that he would add the First Step Act to the Senate's schedule for December.

It’s essential to pass the prison reform legislation before the end of the year, the bill’s proponents have said, because Democrats will take over the House in 2019 and won’t necessarily support the current version of the legislation.

“We’re at the end of a Congress. The alignment is changing. The bill that can be passed now ― who knows whether the same bill will be at the same place six months from now,” said Sen. Chris Coons (D-Del.), one of the bill’s backers.

“If you’ve got a bill that’s good enough, let’s get it done rather than going back to square zero and taking the risk that the president changes his mind, that the House majority won’t accept the same deal, and that this majority won’t accept the same deal,” he said.

The bill picked up serious momentum in the last week as several prominent tough-on-crime conservatives endorsed it, including Sens. Ted Cruz of Texas and David Perdue of Georgia. Fox Broadcasting Co. also issued a statement in support of the package.

The bill’s supporters have said it could easily pass the Senate if only McConnell would allow a vote.

A version of the legislation passed the House earlier this year with a 360-59 vote. The core of the bill is a new pre-release custody program that would give federal prison officials more discretion to release inmates who they believe have a low risk of committing future crimes. (The vast majority of prisoners in the U.S. are in state facilities, not federal ones.)

The Senate version of the bill also would reduce some criminal sentences, including a provision that would allow federal crack cocaine offenders to get the benefit of a 2010 law that made crack sentences more proportionate to regular cocaine sentences. Previously, a 1986 law imposed the same penalty for offenses involving 5 grams of crack as 500 grams of cocaine, a punishment that overwhelmingly disadvantaged African-Americans.

Certain offenders ― such as murderers and child rapists ― would be ineligible for the new program. Cotton has said the bill’s authors omitted too many types of offenders from the list of exclusions, though lawmakers have said they’re trying to make changes to appease the bill’s critics.

“I plan on letting my colleagues decide whether they want to let violent, repeat, serious felons out of prison early,” Cotton told HuffPost Tuesday in response to a question about whether he would try to impede the bill on the Senate floor.

Asked if he wanted to be in Washington for the entirety of the month of December, Cotton said, “Merry Christmas.”

This story has been updated with new quotes from Sen. Tom Cotton.