Sens. Charles E. Grassley (R-Iowa), left, and Richard J. Durbin (D-Ill.) hold a news conference at the Capitol in April 2016. (Jonathan Ernst/Reuters)

A bipartisan group of senators on Wednesday unveiled the latest attempt to reduce the length of mandatory prison terms for drug offenders, a move that directly challenges Attorney General Jeff Sessions’s ongoing efforts to wage a new war on drugs by pursuing maximum possible penalties.

The joint effort, led by Sens. Charles E. Grassley (R-Iowa) and Richard J. Durbin (D-Ill.), would reduce the length of mandatory minimum sentences for repeat nonviolent drug offenses and eliminate the “three strike” provision that requires a life sentence. It also expands the conditions under which judges can elect to take advantage of “safety valves” that let them issue sentences shorter than mandatory minimums for low-level crimes and clarifies that mandatory sentences for firearms used during a drug crime apply only to repeat offenders.

Not every aspect of the bill is a reduction in sentencing. It also issues a handful of new mandatory minimum sentences for domestic abusers who cross state lines, people who export arms to blacklisted countries or terrorist groups, and dealers and traffickers of heroin-laced fentanyl, a synthetic opioid that is significantly more potent — and deadly.

The terms of the criminal-justice bill are largely the same as legislation Grassley and Durbin introduced last year that drew 37 Senate co-sponsors, 17 of them Republicans. But Senate Majority Leader Mitch McConnell (R-Ky.), under pressure from then-Sen. Sessions and others, never put it on the floor for a vote.

Now Sessions is expected to angle against the legislation from his perch in the Trump administration, where earlier this year in a memo he ordered federal prosecutors to seek "the most serious, readily provable offense . . . those that carry the most substantial guidelines sentence, including mandatory minimum sentences."

“Drug trafficking is an inherently dangerous and violent business,” Sessions said at the time.

Sessions said that his order was meant to focus resources on the most hardened criminals. But the directive’s blanket approach effectively upended an Obama-era policy of avoiding pursuing charges for certain criminals that would result in long prison sentences.

Faith in mandatory minimum sentencing was more widely held during the height of the war on drugs in the 1980s and 1990s. But in more recent years, members of both political parties have argued that meting out the toughest possible punishment to all offenders, even for low-level drug crimes, is filling jails with people who do not need to be incarcerated, at a high cost to the public, financially and socially.

Members of the Senate Judiciary Committee have spent five years hammering out the specifics of the criminal-justice bill that was reintroduced Wednesday.

“Mandatory minimum sentences were once seen as a strong deterrent. In reality they have too often been unfair, fiscally irresponsible and a threat to public safety,” Durbin said in a statement accompanying the release of the bill. “Given tight budgets and overcrowded prison cells, our country must reform these outdated and ineffective laws that have cost American taxpayers billions of dollars.”

The reach of the Senate’s bill is limited. It would apply only to federal courts, while the bulk of low-level drug offenses are prosecuted in state courts. Should the legislation pass, however, it will be a pivotal moment for sentencing-overhaul efforts nationally.

For now, senators backing the bill are working to build their list of sponsors to impress on leaders — particularly McConnell — that the effort has a critical mass of support and can pass.

They are also inviting input from the administration. Thus far, Sessions has not been the Trump administration’s point person on criminal-justice policies — instead, senators have been speaking periodically with the president’s son-in-law and senior adviser Jared Kushner about the legislation. Formal hearings on the legislation are also expected, though none have been scheduled.

“This bipartisan compromise ensures that these consequences fit their crimes by targeting violent and career criminals who prey on the innocent while giving nonviolent offenders with minimal criminal histories a better chance to become productive members of society,” Grassley said in a statement released with the bill. “This bill strikes the right balance of improving public safety and ensuring fairness in the criminal justice system.”

Bill sponsors include Sens. Mike Lee (R-Utah), Sheldon Whitehouse (D-R.I.), Lindsey O. Graham (R-S.C.), Patrick J. Leahy (D-Vt.), Jeff Flake (R-Ariz.), Cory Booker (D-N.J.), Tim Scott (R-S.C.), Dianne Feinstein (D-Calif.) and Roy Blunt (R-Mo.).

Sari Horwitz contributed to this report.