WASHINGTON — President Trump was expected to lend his support to a substantial rewrite of the nation’s prison and sentencing laws at an event at the White House on Wednesday afternoon, but opponents of the proposal were making a last-ditch attempt to dissuade him from signing on.

Senators from both parties who hashed out the legislative compromise, which would invest heavily in anti-recidivism programs and lower some mandatory minimum sentences, have staked the bill’s success on Mr. Trump. The president’s support could give political cover to Republicans wary of appearing to reduce some hard-line sentencing rules for drug and other offenses. With his endorsement, they believe they could assemble a coalition of moderate Republicans and Democrats in time to move legislation before the year’s end — and before the new, divided Congress is seated.

Jared Kushner, the president’s son-in-law and the leading advocate of the criminal justice package within the White House, presented the tentative deal to Mr. Trump on Tuesday. Mr. Trump was initially noncommital but later offered a firmer yes, according to administration and congressional officials briefed on the meeting. He is expected to appear midafternoon on Wednesday to announce his support.

The fact that the White House had not yet scheduled the announcement was an indication that the famously mercurial president could change his mind, people close to the process said. Senators and outside advocates for the changes were fretting about that possibility Wednesday morning.

The tentative legislative package builds on a prison reform bill passed overwhelmingly by the House earlier this year by adding changes that would begin to unwind some of the tough-on-crime federal policies of the 1980s and 1990s — which have incarcerated African-Americans at much higher rates than white offenders.

The changes include shortening mandatory minimum sentences for some nonviolent drug offenses and changing the “three strikes” penalty from life in prison to 25 years. They would give judges greater ability to use so-called safety valves to sidestep mandatory minimums in some cases. And the bill would eliminate the so-called stacking regulation that makes it a federal crime to possess a firearm while committing another crime, like a drug offense.

It would also extend retroactively a reduction in the sentencing disparity between crack and powder cocaine signed into law in 2010, which could affect thousands of drug offenders serving lengthy sentences for crack-cocaine offenses that were dealt with far more harshly than the same crimes involving powder cocaine. That disparity hit black Americans hard while letting many white drug dealers off with lighter punishments.

The changes have attracted a broad and unusual group of supporters, such as the billionaire conservative brothers Charles G. and David H. Koch and the American Civil Liberties Union. But powerful pockets of opposition remain among some law enforcement officials and conservative lawmakers like Senator Tom Cotton of Arkansas.

If Mr. Trump follows through, Senator Charles E. Grassley of Iowa, a Republican and chairman of the Senate Judiciary Committee, is expected to quickly introduce the legislation and ramp up his lobbying efforts. Senator Richard J. Durbin of Illinois, the No. 2 Democrat who has worked closely with Mr. Grassley on the issue, would be responsible for assuring Democrats that the bill’s sentencing changes were a deal worth accepting, despite some concessions from an earlier Obama-era effort.

Senator Mitch McConnell of Kentucky, the majority leader, previously told senators backing the plan that he would bring it to the floor for a vote if they had at least 60 senators supporting it. Speaker Paul D. Ryan of Wisconsin, a vocal advocate of such changes, committed to putting the compromise on the House floor in a lame-duck session that began Tuesday if Mr. Trump endorses it and it can clear the Senate.

That may not be easy, especially with so little legislative time to move a complicated bill with broad implications for the nation’s criminal justice system. As of Wednesday morning, many senators had not yet even seen draft text of the bill.

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