The letter to Majority Leader Mitch McConnell, R-Ky., and Minority Leader Harry Reid, D-Nev., seeks to counter concerns about the bill (S 2123) and instead focus on improvements it makes to the corrections system.

“Otherwise, good policy reforms could easily fall victim to politics and fear,” the letter states.

Signers include Michael Mukasey, an attorney general under President George W. Bush, former FBI directors Louis J. Freeh and William S. Sessions, several former U.S. attorneys and several federal appeals court and district court judges.

The letter arrives at the Senate a week after President Barack Obama used his State of the Union address to note bipartisan support for criminal justice legislation.

The compromise bill, negotiated for weeks behind closed doors, has the support of many of the most powerful senators and advanced out of the Senate Judiciary Committee on a 15-5 vote in October.

The bill would give judges more discretion in sentencing, reduce some of what advocates of change consider the most unfair mandatory minimum sentences and allow low-risk prisoners to qualify for earlier release.

McConnell, who makes the decision about floor votes, has not said if the Senate will vote on the bill. Majority Whip John Cornyn, R-Texas, a co-sponsor, has said that would happen in 2016. Judiciary Committee Chairman Sen. Charles E. Grassley, R-Iowa, is also pressing for a vote soon.

The momentum for a sentencing overhaul bill faces a challenge because of the tight schedule in an election year and the possibility that the Republicans won’t retain their Senate majority. There is also opposition, including from a separate group of former federal prosecutors who sent a letter to leadership in December with concerns about the bill.

The letter sent by Mukasey and others Tuesday seeks to counter those concerns. It says the bill makes “modest, reasonable changes” that would amend “just a few sentencing policies that produced unintended consequences and created imbalance in the scales of justice.”

The bill ties longer mandatory minimum prison sentences to high-level drug traffickers and violent criminals, gives prosecutors new tools to seek enhanced penalties for violent criminals and gives federal prisons a way to make the public safer by reducing the number of inmates who commit crimes once released from their sentence.

“A drug dealer using a gun will still be subject to a significant mandatory minimum sentence for use of the firearm plus additional time for the underlying drug offense,” the letter states. “And since the Department of Justice has committed to a case-by-case review to ensure that any resentencing is done carefully and with complete transparency, offenders who pose a threat to public safety will not be released early.”

Holly Harris, executive director of the U.S. Justice Action Network, said in a statement that the bill’s endorsement by the letter signers “speaks volumes about what is needed to reform our criminal justice system and ensure public safety.”

“These well-respected prosecutors and judges understand what it takes to keep our country safe. They have first hand experience of the tools the government needs to put dangerous criminals away and protect our communities,” Harris said.