Senator Mitch McConnell of Kentucky reversed course on Tuesday and said that the Senate would vote on a criminal justice bill before the end of the year, stiff-arming some of his hard-line conservatives and teeing up a bipartisan policy achievement that has eluded lawmakers for years.

Advocates of the proposed prison and sentencing law changes on Capitol Hill and in the White House have spent weeks lobbying Mr. McConnell, the majority leader who controls the Senate calendar. Crucially, they had the backing of President Trump, who urged Mr. McConnell in recent days to “go for it Mitch!” — offering cover to conservative lawmakers who were reluctant to roll back some of the tough-on-crime federal policies of the 1980s and ’90s.

Now proponents of the bill, called the First Step Act — a scaled-back version of earlier efforts — believe the changes could receive as many as 85 votes in the Senate. Speaker Paul D. Ryan of Wisconsin, who also privately pressured Mr. McConnell to take up the bill, has pledged swift action before the House leaves town for the year-end holidays.

In exchange for his backing, Mr. Trump is on the cusp of claiming a rare bipartisan legislative achievement, a welcome victory after voters chose to end complete Republican control of Washington. In it, he could find a model for working with Democrats, who assume the House majority in January.

Senator Richard J. Durbin, Democrat of Illinois, one of the bill’s authors.

“When you have a president do something that seems out of political character, it can sometimes make a historic difference,” said Senator Richard J. Durbin, Democrat of Illinois, one of the bill’s authors.

Broadly speaking, the latest bill pairs investments in anti-recidivism programs in federal prisons and an expansion of early release credits with changes to sentencing laws. Its authors have worked intently on a set of yet-to-be-released revisions in recent days to accommodate wavering Republicans. Still, the bill is expected to have an impact on thousands of current federal inmates and future offenders. State laws and prisons will not be affected.

Many of the changes appeared to be devised to limit the types of offenders who would be eligible for early release credits under the law, according to senators and congressional aides. Senator Charles E. Grassley, Republican of Iowa and a lead author of the bill with Mr. Durbin and Senator Mike Lee of Utah, said the changes had been cleared with Democrats.

In an era of partisan gridlock in Washington and deepening policy divisions among conservatives and liberals, the push to overhaul the nation’s criminal justice system has stood out as a rare shared objective. A broad and unusual coalition of outside advocates — including the American Civil Liberties Union and the Fraternal Order of Police — has emerged in recent years to push lawmakers to confront a system all sides agree is costly and, in may cases, unfair.

A number of states have already made similar changes to their own laws, and proponents of the congressional legislation hope that changes to the federal system could in turn spur state-level changes that would affect a far larger prison population.

The bill that will most likely pass the Senate next week is more modest than a bipartisan measure introduced in the waning years of the Obama administration. That legislation would have affected more people by making most sentencing changes retroactive — something liberals had to concede was not possible in the Trump era.

Still, the looming vote will underscore the significant shift in public opinion and policymakers’ views over the last two decades, away from the policies of an era when the federal government declared war on drugs and crime toward what lawmakers like to call a “smart on crime” approach.

“We made some decisions 25 years ago here that were wrong,” Mr. Durbin said. “We imprisoned thousands of people, mainly African-Americans, for sentences that made no sense in light of their crimes. What we have tried to do is repair that damage and be smarter in the way we sentence.”

Before Tuesday, Mr. McConnell had repeatedly said that there was most likely not enough time to consider the measure this year, and Republican leaders maintained as recently as a few days ago that the bill did not have the support of the majority of Republicans. Proponents feared that without a vote this year, the deal would have fallen apart in the new Congress because Democrats would demand more liberal changes to sentencing laws.

Senators and White House officials involved in the effort said a slow build of support among Republicans, coupled with steady pressure from the White House, finally reached a tipping point on Monday, they said.

Mr. Grassley said that Mr. McConnell’s change of course amounted to “a realization that the president wanted it,” and the majority leader of the Senate, “if he’s also of the president’s party, has traditionally delivered on the president’s programs.”

In a rare show of comity from a Democrat, Mr. Durbin explicitly gave credit to Mr. Trump for giving Republicans the push they needed to secure a vote.

Mr. McConnell made clear on Tuesday that the Senate was considering the legislation “at the request of the president” and said that debate could begin later this week. With the addition of new business, Mr. McConnell also held out something of a threat: If senators were not willing to expedite consideration of some issues before the Senate, the chamber could hold votes the week between Christmas and New Year’s Day.

In a sign of growing Republican support, Mr. McConnell’s chief lieutenant, Senator John Cornyn of Texas, quickly seized on the news to announce that he, too, would vote for the bill. He had been accused by some fellow Republicans and outside conservative advocacy groups of quietly trying to kill their efforts.

Still, strong opposition remained among some conservatives. Senator Tom Cotton, Republican of Arkansas, said he was prepared to try to amend the bill on the Senate floor “to address its many remaining threats.”

“Unfortunately, the bill still has major problems and allows early release for many categories of serious, violent criminals,” he said in a statement.

The bill’s authors say that these fears are overheated and that provisions written into the legislation will ensure that in practice, truly dangerous or violent inmates will not be released early.

Senator Cory Booker, a New Jersey Democrat

Democrats are seeking to shore up their left flank. Senator Cory Booker, a New Jersey Democrat who is one of only three African-American senators, said fellow progressives should realize the bill would overwhelmingly help African-Americans.

“I would be deeply disappointed if anybody in our caucus votes against a bill that disproportionately helps low-income people and minorities, literally gives them pathways to liberation,” he said.

SOURCE: New York Times