The push for criminal justice reform, the legislation seen as having the best chance of passing Congress this session, has stalled as key senators hold slim hopes of passing anything before the end of year.

Senate Judiciary Committee Chairman Chuck Grassley, R-Iowa, who played a key role in negotiating the compromise that ultimately became the Sentencing Reform and Corrections Act, said in an interview Wednesday the bill is stalled for now.

“I don’t see how it gets done before” July 15, Grassley said, referencing the day the senators depart from Washington and won't return until after Labor Day. “It’s a real big disappointment to me because we’ve worked so hard to do what the leadership wanted to get out more Republican sponsors."

Sen. Dick Durbin of Illinois, the second-ranking Democrat in the chamber and an author of the justice reform bill, said Republicans had offered him “little to no hope” that the legislation would move forward. He called it a “missed opportunity.”

Texas Sen. John Cornyn, the Republican whip and a lead sponsor of the measure, said he’d hoped the House would move more quickly and provide momentum in the Senate, but “apparently we ran out of time.”

Time will be in short supply the rest of the year. After its summer recess, Congress will be in session for five weeks in September and October. Then lawmakers leave again until after the election, when they will only have a few weeks to wrap up end-of-the-year matters.

Asked if he held out hope that a reform measure could pass this fall, Cornyn said he was optimistic, but that “obviously, the clock is not our friend, so it’s going to be a challenge.”

Overhauling criminal justice laws, which has support across the ideological spectrum, was at the top of the to-do list for President Obama and many lawmakers. But the effort has lost momentum in recent weeks.

In his second term, Obama has worked quietly with lawmakers from both parties to encourage passage of justice reform legislation and has touted its benefits for more than a year.

Obama began the year optimistic that sentencing reforms, along with ratification of the Trans-Pacific Partnership trade pact, enjoyed bipartisan support and could become legacy-burnishing legislative achievements on which to end his presidency. It is now more likely that he could wind up empty-handed, bequeathing trade and justice debates to his successor.

Valerie Jarrett, senior adviser to Obama, told RealClearPolitics the president is "disappointed" Congress will leave Washington without acting this summer.

"While the President and his Administration will continue to work to secure passage of meaningful criminal justice reform legislation -- reforms which would enhance public safety and save money and already have bipartisan support from Members of Congress, law enforcement, veterans, faith and civil rights groups -- we are disappointed that it appears that Congress is poised to leave for the summer without acting on this historic opportunity,” Jarrett told RCP in a statement.

The United States spends an estimated $80 billion a year to incarcerate criminals. Obama recently pointed to savings, safety and justice as rationales for reform legislation while addressing a medal of valor ceremony for police and public safety officers at the White House.

“We’re going to keep pushing Congress to move forward in a bipartisan way to make our criminal justice system fairer and smarter and more cost-effective and enhance public safety and ensure the men and women in this room have the ability to enforce the law and keep their communities safe,” he said.

To drive home his conviction that incarceration hurts prisoners’ children, families and communities, Obama commuted the sentences of 348 non-violent drug offenders. He met with some of the offenders, and was the first sitting president to visit a federal cellblock last summer.

Momentum for the justice reform legislation moved slowly over the last 18 months as lawmakers sought to compromise. Durbin and conservative Republican Sen. Mike Lee of Utah sponsored legislation that would have reduced certain mandatory minimum sentences for non-violent drug offenses, while Cornyn and Democratic Sen. Sheldon Whitehouse of Rhode Island sponsored legislation establishing programs to reduce the number of repeat offenders.

In October, legislation was introduced that combined the two issues – it would reduce some mandatory minimum sentences while giving judges more discretion over sentencing, and also would create new recidivism programs to allow some non-violent offenders to earn reduced sentences.

The legislation passed in committee, but several Republican senators complained it was too lenient on violent criminals. The senators worked to assuage those concerns and announced a new compromise in May, but it has gone nowhere in the last month, with Majority Leader Mitch McConnell declining to put it on the Senate floor, where appropriations bills, partisan warfare over gun control and efforts to combat the Zika virus, among other issues, have taken priority.

Durbin directly blamed McConnell for the lack of movement, saying that the majority leader had “five Republican senators vocally, publicly opposing it, and he didn’t want to take them on."

David Popp, a spokesman for McConnell, said in a statement, “"Discussions continue within our Conference on the issue."

Senators have said recently they hoped the House would pass legislation to kick-start Senate action.

In the House, however, the Judiciary Committee has moved at a slower pace on a package of justice-system reform bills. A Republican committee aide told RCP the House has passed nine bills dealing with sentencing and prison reform, over-criminalization and asset forfeiture. The committee is considering four remaining justice reform bills, which supporters hope will pass there and come to the House floor as a package. It’s uncertain whether that will happen before the summer recess.

“The goal is to wrap up our initiative here in the committee level before we head out to the recess,” the committee aide said. “That’s not confirmed at this time, but we’re hopeful that’s what we could do."

Grassley expressed some optimism, saying there was a “good chance” the legislation could pass in the fall, either in the five weeks after Labor Day or in the lame-duck session after the election in November. But he said that decision is up to McConnell.

“There’s one person that decides what the agenda of the United States Senate is,” Grassley said, adding that he thought McConnell would “give fair consideration to it."

Still, even if the timing works out for both chambers to pass legislation before the election or after a new president is elected, there are policy disagreements that could prevent anything from becoming law.

Republicans insist there must be reforms to mens rea, Latin for “guilty mind,” the principle that a person must have some level of criminal intent to be found guilty of a crime. A version of that reform is in the House legislation, but not in the Senate version. Rep. Bob Goodlatte, R-Va., chairman of the House Judiciary Committee, has said mens rea reform must be attached to any criminal justice legislation.

Sen. Orrin Hatch, R-Utah, a former chairman of the Judiciary Committee, has been pushing for the mens rea legislation to be included in the Senate’s version of the justice reform.

“All they have to do is put a mens rea clause in there and it would get done,” Hatch said. “All they’ve got to do is accept my language and I would certainly weigh in and help them to pass the bill.”

Democrats, however, strongly object to including that proposal. Their opposition stems in part from the belief that it would create a difficult standard for prosecutors and could make it easier for some who commit white-collar or corporate crimes to avoid prosecution.

Hatch “obviously has his own agenda, his own bill, his own issue, and he’s trying to hitch a ride,” Durbin said. “If his mens rea bill is as good as he thinks it is, it will stand on its own. It really has no relationship to what we’re trying to achieve here with criminal justice reform.”

Whitehouse, one of the Democratic senators who co-authored the justice reform legislation, blamed the Koch brothers, who have been outspoken in pushing for reforms to the justice system. Whitehouse suggested they only backed justice reform as a way to push mens rea, and said it has “totally tanked” the bipartisan effort.

“They figured they could get something moving with their right hand and then stuff their corporate protection mens rea reform in, once it got going, with their left hand,” Whitehouse told RCP. “I think that’s a fatal poison pill on what had been a good faith effort between Republicans and Democrats.”

He said the legislation was “dead as a doornail unless the Koch brothers say we’re not going to insist on mens rea reform and get out of everybody’s way. But they pull the chains of almost every Republican around here so it makes it hard to go forward without their blessing. Particularly the leadership folks who are dependent on the Koch brothers for all of the party money."

Cornyn, who co-authored his portion of the bipartisan legislation with Whitehouse, said the Democratic senator’s criticism was unfair.

“I think he’s sort of putting the cart ahead of the horse,” Cornyn said, pointing out the mens rea reform is not part of the Senate bill, though he supports efforts to add it into a final package. “I don’t see that as an obstacle of actually getting something done."

Mark Holden, general counsel at Koch Industries, said Whitehouse was “misinformed” and that his criticism of the Koch brothers was “completely inaccurate.” Holden said that while they wanted comprehensive criminal justice reform that would include mens rea reforms akin to what's in the House bill, they support both the House and Senate efforts and wanted some form of the legislation to pass, even if it doesn’t ultimately include mens rea.

"We want to see something happen and we’ve been saying that for over a year now,” Holden said. “If Congress can’t agree, if the House bill doesn’t survive, that’s up to Congress, not up to Koch. I’m hopeful that the House will pass their legislation, the Senate will pass their legislation and they’ll work it out.”

Given all the problems ahead for justice reform, Durbin was pessimistic that anything would happen this year, saying it’s a “long shot” that something might move in the weeks after the election. He held out some hope for the future though, and indicated that he thought the effort could be revived if Democrats won back the Senate majority in November.

“It’s the end of this Congress and we have to start over,” Durbin said. “Maybe with a Democratic majority in the Senate it will be better."

Cornyn, too, shared optimism that no action this year did not necessarily mean the end of criminal justice reform in Congress.

“Fortunately, this is bipartisan and so I think regardless of who’s elected president and who’s in the majority and who’s in the minority, we can still get this done next year if we can’t get it done before then,” Cornyn said.

White House Reporter Alexis Simendinger contributed to this report.