For example, in the case of a defendant accused of conspiring to sell five kilograms of cocaine — an amount that would set off a 10-year mandatory minimum sentence — the prosecutor would write that “the defendant conspired to distribute cocaine” without saying how much. The quantity would still factor in when prosecutors and judges consult sentencing guidelines, but depending on the circumstances, the result could be a sentence of less than the 10 years called for by the mandatory minimum law, the official said.

It is not clear whether current cases that have not yet been adjudicated would be recharged because of the new policy.

Amid a rise in crime rates a generation ago, state and federal lawmakers began passing a series of “tough on crime” laws, including mandatory minimum sentences for drug possession. But as crime rates have plummeted to 40-year lows and reduced the political potency of the fear of crime, fiscal pressures from the exploding cost of building and maintaining prisons have prompted states to find alternatives to incarceration.

Driven in part by a need to save money, several conservative-leaning states like Texas and Arkansas have experimented with finding ways to incarcerate fewer low-level drug offenders. The answers have included reducing prison terms for them or diverting them into treatment programs, releasing elderly or well-behaved inmates early, and expanding job training and re-entry programs.

The policy is seen as successful across the ideological divide. For example, in Texas, which was an early innovator, taxpayers have saved hundreds of millions of dollars on what had been projected as a need to build prison space. With crime rates remaining at generational lows, the space is no longer necessary.

Several years ago, a group called Right on Crime formed to push what it calls the “conservative case for reform.” Its Republican affiliates include Jeb Bush, a former Florida governor; Edwin R. Meese III, an attorney general during the Reagan administration; and Newt Gingrich, a former House speaker.

“While the federal prison system has continued to slowly expand, significant state-level reductions have led to three consecutive years of decline in America’s overall prison population — including, in 2012, the largest drop ever experienced in a single year,” Mr. Holder’s speech said. “Clearly, these strategies can work. They’ve attracted overwhelming, bipartisan support in ‘red states’ as well as ‘blue states.’ And it’s past time for others to take notice.”