Attorney General Jeff Sessions has directed federal prosecutors to charge suspects with the most serious offenses they can prove.

WASHINGTON — Attorney General Jeff Sessions, who has promised to make combating violence and drugs the Justice Department’s top priority, is directing federal prosecutors to pursue the most serious charges possible against the vast majority of suspects, a reversal of Obama-era policies that is sure to send more people to prison and for much longer terms.

“This policy affirms our responsibility to enforce the law, is moral and just, and produces consistency,” Sessions wrote in a memo sent Thursday to U.S. attorneys and made public today. The move amounts to an undoing of Obama administration criminal justice policies that aimed to ease overcrowding in federal prisons and contributed to a rethinking of how drug criminals were prosecuted and sentenced. Critics said the change will subject more lower-level offenders to unfairly harsh mandatory minimum sentences.

Sessions contends a spike in violence in some big cities and the nation’s opioid epidemic show the need for a return to tougher tactics.

“The opioid and heroin epidemic is a contributor to the recent surge of violent crime in America,” Sessions said in remarks prepared for a Thursday speech in Charleston, W.Va. “Drug trafficking is an inherently violent business. If you want to collect a drug debt, you can’t, and don’t, file a lawsuit in court. You collect it by the barrel of a gun.”

The policy memo says prosecutors should “charge and pursue the most serious, readily provable offense” — something more likely to trigger mandatory minimum sentences. Those rules limit a judge’s discretion and are typically dictated, for example, by the quantity of drugs involved in a crime. The memo concedes there will be cases in which “good judgment” will warrant a prosecutor to veer from that rule. But any exceptions will need to be approved by top supervisors, and the reasons must be documented, allowing the Justice Department to track the handling of such cases by its 94 U.S. attorney’s offices.

The requirements “place great confidence in our prosecutors and supervisors to apply them in a thoughtful and disciplined manner, with the goal of achieving just and consistent results in federal cases,” the memo states.

The directive rescinds guidance by Sessions’ Democratic predecessor, Eric Holder, who told prosecutors they could in some cases leave drug quantities out of charging documents so as not to trigger long sentences.