US President Barack Obama speaks at the NAACP's annual convention in Philadelphia July 14, 2015. [Photo/Agencies]

The White House said Obama wouldn't hesitate to commute more sentences in the coming months if the circumstances were right. Yet Obama's ability to address the problem unilaterally is limited, as the White House readily concedes. So Obama has set his sights on the kind of comprehensive fix that only Congress can provide.

Working in Obama's favor: tentative but optimistic signs of common ground between Republicans and Democrats.

Republicans in particular have spoken with growing enthusiasm about the need for structural change. Senate Judiciary Committee Chairman Chuck Grassley has been working on legislation that could reduce some mandatory minimums. Republican Sen. John Cornyn and Democratic Sen. Sheldon Whitehouse are backing a bill that would steer lower-risk inmates into programs where they could earn earlier release by participating in recidivism-reduction programs.

In another positive sign for the prospects of justice reform, a number of 2016 presidential candidates have taken an active interest in the issue. Sen. Rand Paul, a Kentucky Republican, has mounted a vocal push to restore voting rights to nonviolent felons who have served their terms and to make it easier for people with criminal records to get jobs.

But not all Republicans were receptive to Obama's pitch. A group of 19 Republicans, led by House Judiciary Committee Chairman Bob Goodlatte, wrote a letter Tuesday to Attorney General Loretta Lynch accusing Obama of blatantly usurping congressional authority and using his pardon power for political purposes.

Since Congress enacted mandatory minimum sentences for drug crimes, the federal prison population has multiplied, from just 24,000 in the 1980s to more than 214,000, according to Families Against Mandatory Minimums. In 2010, Obama signed the Fair Sentencing Act, cutting penalties for crack cocaine offenses. And last year, the independent Sentencing Commission reduced guideline ranges for drug crimes and applied those retroactively.