WASHINGTON - New Jersey Sen. Cory A. Booker joined a bipartisan coalition Thursday to unveil a highly anticipated criminal justice bill - one that, if passed, would represent a landmark in his young Senate career.

The package, with support across the political spectrum, has a chance to be one of the few significant new initiatives to become law in a bitter political atmosphere.

It also represents a step toward Booker's prime goal since joining the Senate: easing mandatory minimum sentences and providing second chances for people after they leave prison, particularly juveniles.

"After years of the justice system being broken, we're announcing a solid step forward toward reform," Booker, a Democrat, said in an interview. The bill does not include everything he had hoped for, but he said it represented "a number of really good strides on a much longer journey."

Booker joined four Democrats and four Republicans Thursday to announce the long-awaited compromise. Several said the hard-won deal showed how Congress should work.

"There are things in here that each of us like. There are items that each of us would rather do without," said Sen. Charles E. Grassley (R., Iowa), who was skeptical but won over after the reformers agreed to toughen some sentences.

Support from Grassley, as chairman of the Judiciary Committee, was vital to the bipartisan deal.

The effort has backing from liberals who have pushed to ease penalties and help convicts turn around and from conservatives who want to cut the costs of incarceration and see a moral impetus for rehabilitation. The initiative has been pushed by the Republican megadonor Koch brothers, antitax crusader Grover Norquist, and the White House.

The range of views was obvious at the news conference. Grassley and another Republican, Texas Sen. John Cornyn, said they were glad mandatory minimum sentences would remain in place. Vermont Sen. Patrick J. Leahy, a senior Democrat, said the minimums should be eliminated.

The final bill would cut some mandatory minimums, mainly for gun crimes or nonviolent drug offenses. Mandatory life sentences under the "three-strikes" law would drop to 25 years; 20-year minimums would fall to 15.

Some new crimes - domestic violence and terrorism - would trigger mandatory sentences, and 10- and five-year minimum sentences would remain unchanged.

Judges, however, would get more discretion to ease punishments in some cases that normally require a 10-year sentence. The changes could be retroactive.

The measure also includes steps to bolster rehabilitation programs and expunge juvenile records - a step Booker prioritized to help job-seekers after youthful troubles.

Putting the bill together was "like a Rubik's cube," said Sen. Charles E. Schumer (D., N.Y.). "So many different pieces, one turns one way, the other turns the other way."

Booker's support was sealed on the Senate floor Wednesday with a handshake with Grassley. He won the expungement provision, but with limits: Juveniles tried as adults or convicted of domestic violence won't be eligible.

Booker, one of the Senate's most skilled orators, added a personal touch, relaying how he had seen people treated differently in the affluent suburb where he grew up and at Stanford University, where he went to college, compared with those he met as mayor of Newark.

Schumer called Booker "the conscience" of the effort.

"Time and time again he got up in our caucus lunches and made heartfelt pleas that we should move forward on this that moved everybody," Schumer said.

Despite the bill's bipartisan support, the fractious Senate and House could still present hurdles. Proposals have been criticized by prosecutors' organizations, who say mandatory minimums ensure fair sentences, allow authorities to apply pressure on suspects, and keep citizens safe.

The plan also has limits: It would affect only federal prisons, where Booker said the population has grown by 800 percent since the 1980s, but which account for just 13 percent of the U.S. prison population, according to John Pfaff, a professor of criminal and sentencing law at Fordham University.

About 6,500 inmates would be eligible for release due to retroactive sentencing changes, according to Grassley's office. Lawmakers did not have an estimate Thursday of the bill's financial impact.

Some provisions seem symbolic. The new mandatory minimums would affect few federal cases - less than 1 percent of current federal inmates have been convicted of interstate domestic violence or terrorism, Pfaff said.

And while Booker won limits on the solitary confinement of juveniles, Pfaff said only 33 juveniles are in the federal system, none in solitary.

With such a limited slice of the prison population affected by federal courts, the biggest changes would have to come from states, he said.

Booker hoped a new federal law would spur such action.