WASHINGTON -- Several organizations representing state and local law enforcement are quietly trying to kill a bipartisan bill that would roll back tough mandatory sentences for people convicted of federal drug offenses under legislation passed during the height of America’s drug war three decades ago.

These groups include the National Sheriffs' Association, the International Association of Chiefs of Police, the National Narcotic Officers' Associations' Coalition, the National Association of Police Organizations and the Major County Sheriffs' Association, The Huffington Post has learned.

They hope to weaken congressional support for the Smarter Sentencing Act, which would reform the nation's mandatory minimum statutes, authorizing federal judges to sentence drug defendants to less time behind bars than what current law requires. The legislation passed the Senate Judiciary Committee in January, when, in a rare instance of bipartisan collaboration these days, Republicans Mike Lee of Utah, Ted Cruz of Texas and Jeff Flake of Arizona joined the committee’s Democrats in supporting the measure. Its House counterpart is still sitting in committee.

Republicans and Democrats alike have argued that the bill would allow the federal government to get control of its burgeoning prison budget and free up resources needed for other public safety initiatives, such as programs that help former prisoners find work and housing.

But Bob Bushman, president of the National Narcotic Officers' Associations' Coalition, one of the groups fighting the bill, contends that state and local governments could end up bearing some of those same costs. To compensate for the federal government’s softer approach, he argues, states and counties would be compelled to lock up more people than they do now.

Major drug dealers “need to be locked up somewhere,” Bushman told HuffPost. “Some of these folks have worked hard to get to prison."

Supporters of the bill respond that this argument overlooks an important fact about federal prisons. “We have a lot of people in the federal system now who are not major dealers, people who are convicted of low-level drug, nonviolent crimes who do not deserve long sentences,” said Jesselyn McCurdy, an attorney in the Washington Legislative Office of the American Civil Liberties Union.

A number of law enforcement agencies have already joined advocacy groups like the ACLU in endorsing the bill. They include the Major Cities Chiefs Association, the International Union of Police Associations, the American Correctional Association, the International Community Corrections Association and the American Probation and Parole Association. Attorney General Eric Holder backs the measure as well.

Bushman and his allies, however, aren’t the first law enforcement advocates to speak out against the bill. The Federal Law Enforcement Officers Association and the National Association of Assistant United States Attorneys have also come out against federal sentencing reform in recent months. Unlike Bushman’s cohorts, both of these groups represent officials who work for the federal government, and both have stated their positions in public.

The National Narcotic Officers' Associations' Coalition, the National Sheriffs' Association and the other state and local groups have been working behind the scenes. Several of them had previously lined up against Debo Adegbile, the president's nominee to head the Justice Department's Civil Rights Division, and helped block his confirmation last month.

Lobbyists with the National Association of Police Organizations and other groups visited the offices of Sens. Dianne Feinstein (D-Calif.), Kay Hagan (D-N.C.), Amy Klobuchar (D-Minn.) and John Walsh (D-Mont.) to discuss the groups' opposition to the reform package.

A spokeswoman for the International Association of Chiefs of Police confirmed that the organization was lobbying against changes on Capitol Hill, but said it wasn't prepared to speak publicly on the topic.

Fred Wilson, an official with the National Sheriffs' Association, said his group isn't formally opposed to the legislation in principle but believes the bill needs more study -- even though it has already passed through the Senate Judiciary Committee. "It may be [late], but our legislative folks seem to think not all is lost," Wilson said.

A letter from Bushman and his group to Senate Majority Leader Harry Reid (D-Nev.) and Senate Minority Leader Mitch McConnell (R-Ky.) -- just one of several letters written by the Smarter Sentencing Act opponents that Bushman said are floating around Capitol Hill -- argues that federal policy should not be driven by "second-order effects of America’s drug problem" like incarceration costs.

If Bushman and his allies manage to undermine the bill, it won’t be the first time that members of law enforcement have successfully scaled down efforts to reform federal sentencing.

Several law enforcement organizations opposed the Fair Sentencing Act of 2010, which reduced the disparity between mandatory minimum sentences for crack and powder cocaine. Under the previous law, the disproportionately large numbers of black men arrested for crack offenses faced far harsher sentences than their white counterparts, who were more likely to be arrested for offenses involving powder cocaine. Initially, some law enforcement groups suggested increasing the penalties for powder cocaine, rather than decreasing those for crack. Although that didn't happen, their opposition ultimately led to a compromise bill that significantly reduced, but didn't eliminate, the sentencing disparity.

Bushman said it was "a little early" to talk about whether law enforcement groups could be won over with a compromise bill this time, but said members of Congress first need to look at the "broader implications" of rolling back mandatory minimums.

Democratic congressional aides acknowledged that they have been speaking with a number of law enforcement groups about the bill and said they hoped some of the concerns raised would be addressed, but likewise noted it was still relatively early in the legislative process.