Momentum is building on Capitol Hill for legislation to reform the country's prison system, with lawmakers on both sides of the aisle in both chambers working together to craft bills that could land on President Barack Obama's desk before the end of year.

"There are some sincere efforts on the part of some Republicans in Congress to deal with the problems of mandatory minimums in sentencing and rehabilitation and… I think that, wherever I see an opportunity these days, with only 18 months to go, I intend to seize it," Obama told NPR this week, less than a month after he became the first sitting president to visit a federal prison.

Some prison reform advocates caution that this is merely the beginning of what could be a long and thorny path to reducing the country's staggering prison population, which has surpassed 2.2 million inmates, nearly 1 percent of the adult population. But that road will likely lead to a place that politicians are wary of going: Reducing the sentences of violent offenders.

A report released this week by the Urban Institute included an interactive tool that allows state-by-state analysis of how effective various changes to sentencing laws would be in reducing the country's prison population.

It found that the politically popular proposals to ease the sentences of drug offenders, property crime offenders, and other nonviolent offenders would result in only a slightly higher reduction - say, 5 percent - than the 2 percent reduction that would occur if no action were taken. "More than half of people in state prisons are incarcerated for a violent offense, so significantly reducing mass incarceration will require addressing lengths of stay for crimes that are unlikely to be eligible for prison diversion programs," the report said.

The Urban Institute pointed out that even modest changes to violent offenders' sentences would result in a major decrease in the prison population. Reducing the sentences of violent offenders by 15 percent in Michigan, New Jersey, New York, and Rhode Island would result in a much larger drop in the prison population than reducing drug admissions by 50 percent, the report said.

Brian Elderbroom, a senior research associate in sentencing and corrections policy at the Urban Institute, said that the report points out that truly reducing the prison population requires dealing with violent offenders.

Elderbroom praised the bipartisan effort known as the Cut 50 campaign, organized by Newt Gingrich and political commentator Van Jones, and backed in part by the Koch brothers, which seeks to cut the prison population by 50 percent in 10 years. But he told VICE News it will be important for that campaign to realize how effective their legislative proposals really will be on reducing the number of Americans in prison.

'There's an increasing amount of rhetoric but not enough specific solutions.'

"We're excited to see that kind of movement and consensus, but what we're trying to do is see how we can actually do this," he said. "There's an increasing amount of rhetoric but not enough specific solutions. If we're really serious about tackling this we'll have to talk about this population [of violent offenders]."

In order to achieve meaningful cuts to the prison population, such as the 50 percent target of Cut 50, it will take a "combination of policy changes" that reevaluate who ends up behind bars and for how long, Elderbroom said. And to combat the explosion in the prison population over the past two decades, lawmakers will have to roll back policy decisions — often piecemeal — to bring the inmate population back in line with the international community, he said. He noted that while America has about 5 percent of the world's population, it has 25 percent of all prisoners.

"Everything like mandatory minimums, the three-strike law, and all those other bumper sticker criminal justice policies adopted over recent decades… As states increased those requirements, we sent more people to prison and kept them there for longer," he said.

According to Elderbroom, shrinking the prison population will reduce the $50 billion the US spends on state prisons each year, and the $30 billion spent on courts, prosecutors, sheriffs, and local jails. That would free up resources for diversionary programs, job training, and support services, and allow more spending on education and other priorities, he said.

Watch the VICE News documentary, **Institutionalized: Mental Health Behind Bars:**

Jessica Jackson Sloan, the national director of Cut 50, told VICE News she thought legislation altering sentences for violent offenders would be feasible "down the road," if not in the first round of bills targeting criminal justice reform.

"There needs to be a major change in the conversation in America," she said. "Right now, you tell someone I want to let violent criminals go after a shorter amount of time and people freak out. They have a picture of the people who they don't want to get out."

For now, the organization plans to launch a push this fall that will help rally public support for the legislation being proposed in Congress. Cut 50 is mainly focused on preventing offenders from being sentenced to prison by sending them to drug courts, veterans courts, and other diversionary programs, and on changing the sentencing laws for nonviolent offenders, she said.

'Right now, you tell someone I want to let violent criminals go after a shorter amount of time and people freak out.'

"With the Safe Justice act and other legislation we've looked at, we're not looking to let violent offenders out," Jackson Sloan said. "We are looking at using our prison resources for those who actually need it, and to redefine what we mean by violent crime, and we are trying to reallocate resources so that nonviolent offenders aren't entering the system and becoming part of that downward cycle of crime."

Jackson Sloan pointed out that the largest percentage of American prisoners — 1.3 million inmates — are in state prisons, followed by 700,000 in local jails, and about 200,000 in federal prisons, so the greatest effect of federal legislation might be sending a signal to governors and state legislators to take similar action.

Darrell Stephens, the executive director of the Major City Chiefs police organization, said that his group is supportive of criminal justice reform, particularly reducing sentences for drug offenders and those who commit property crimes.

"We do not oppose reducing the mandatory minimums for drug offenses and that kind of thing, we actually think that would be a good thing as long as it was accompanied by drug treatment and programs for reentry," Stephens told VICE News.

As for violent offenders, Stephens said the important thing in any sort of reform will be evaluating each prisoner on a case-by-case basis to ensure that those who are convicted for nonviolent offenses don't have serious violent histories, and that others convicted of violent crimes really deserve their lengthy sentences.

Follow Colleen Curry on Twitter: __@currycolleen