WASHINGTON — The Justice Department wants low-level drug criminals who were sentenced under tough laws from the days of the crack epidemic to ask the president for early release from prison.

In an unprecedented move, Deputy Attorney General James M. Cole asked defense lawyers on Thursday to help the government locate prisoners and encourage them to apply for clemency. The clemency drive is part of the Obama administration’s effort to undo a disparity that flooded the nation’s prison system and disproportionately affected black men.

Offenses involving crack, which was more commonly used in black communities, carried more severe penalties than crimes involving powder cocaine, which was usually favored by affluent white users. In some cases, crack crimes resulted in a 100-to-1 sentencing disparity.

Congress reduced that disparity in 2010. In December, President Obama commuted the sentences of eight federal inmates who received sentences under the old rules.