WASHINGTON (AP) — The share of federal drug offenders subject to mandatory minimum sentences that critics call unduly harsh has plunged in the last year, according to figures Attorney General Eric Holder plans to release Tuesday in arguing for the success of his criminal justice policies.

Preliminary data obtained by The Associated Press show a 6 percent drop in the number of federal drug trafficking prosecutions in the year that ended last September. At the same time, the percentage of drug cases in which prosecutors pursued mandatory minimum sentences dropped from nearly 64 to roughly 51, which the Justice Department says is the lowest on record.

With Holder departing within weeks, the Justice Department is working to measure the impact of criminal justice policies likely to define his legacy. Those include an August 2013 initiative known as "Smart on Crime," which directed prosecutors to avoid charging non-violent drug offenders with crimes that carry mandatory minimum sentences — rigid, usually years-long punishments that are generally dictated by drug quantity and restrict a judge's discretion.

In a speech Tuesday, Holder is expected to disclose new U.S. Sentencing Commission data that he will say show prosecutors generally are following his policy and creating changes in how drug criminals are charged and punished.

"It is having a real and measurable impact on the decisions made by federal prosecutors from coast to coast," Holder will say, according to his prepared remarks. "The changes we've implemented are firmly taking hold. And our key reforms appear to be successful by every measure we've seen so far."

Experts credit Holder for helping raise sentencing policy as a public issue, though some also say it's hard to gauge how much is directly attributable to "Smart on Crime."

"I think it's impossible to identify one factor that is going to be the cause of all the change," said Julie Stewart, president of Families Against Mandatory Minimums. "But I don't think it's wrong to give Holder some credit for caring about who's in prison or trying to get his U.S. Attorneys not to overcharge."

For decades, law enforcement authorities have evaluated success through numbers of convictions and the length of prison terms, conventional measurements seen as particularly useful during the 1980s-era crack epidemic. But in six years as attorney general, Holder helped anchor a movement that sought limits on long drug sentences as a matter of fairness and cost, promoting a sentencing regime based more on the facts of a case and less on formulas.

Crucial to that effort was "Smart on Crime," a cost-cutting initiative that instructed federal prosecutors to stop charging many low-level drug defendants with crimes that carry mandatory minimums and to consider diverting more non-violent offenders away from prison and into treatment. The idea was to encourage prosecutors to focus resources on more dangerous drug criminals, even if that meant bringing fewer federal cases. Between fiscal years 2013 and 2014, the overall number of federal drug prosecutions dropped from 22,215 to 20,824, the data show.

Holder's move caused grumbling among some prosecutors, who worried they'd lose leverage to negotiate plea bargains, and among some law-and-order Republicans on Capitol Hill. But in the last fiscal year the percentage of guilty pleas has held steady, along with the percentage of drug defendants who received credit for cooperation, which the Justice Department says belies the prosecutors' concerns.

Still, it's not clear from the limited snapshot whether the numbers reflect a new normal, or will continue to fluctuate as they have for most of the last decade. In his speech, Holder will say that he expects the trend to continue and that the declining use of mandatory minimum punishments can only be attributed to "Smart on Crime."

Yet it's also true that, even before Holder's policy announcement 18 months ago, long drug sentences were already facing scrutiny in the justice system, with some downward pointing trends starting to take shape. In the last decade, for instance, the U.S. Supreme Court made sentencing guidelines advisory rather than mandatory and President Barack Obama signed a law to cut crack cocaine penalties. Last year, the Sentencing Commission — an independent panel that sets sentencing policy — slashed guideline ranges for drug crimes and applied those changes retroactively.

The new data do not provide a geographic breakdown, making it hard to tell how evenly the changes are being carried out across the country. But anecdotal reports from defense lawyers suggest some prosecutors are taking a different approach.

"I think it's working really, really well," said Chicago attorney Joseph Lopez. "I see significant decreases. They're going down from 10 years to three and four years. It's huge."

But Katherian Roe, the federal defender in Minnesota, said she thinks that while prosecutors are trying to be mindful of Holder's guidance, "There are still cases out there where they're charging mandatory minimums where it seems, under the policy, perhaps inappropriate."

In his speech, Holder will argue that the data show "a potential paradigm shift" in how the country approaches justice and for a Justice Department that, one decade ago, instructed prosecutors to seek the most severe punishment possible for drug cases.

"While old habits are hard to break, these numbers show that a dramatic shift is underway in the mindset of prosecutors handling nonviolent drug offenses," he will say.

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