When President Obama, the Koch brothers, the American Civil Liberties Union and Newt Gingrich all agree on an issue, you know that something important may be happening.

And you also know that there must be a catch, or maybe three.

“Criminal justice reform” — cutting back on a rate of incarceration that jumped fourfold in four decades — has become a bipartisan buzzword. Many people of different political stripes agree that too many Americans are being imprisoned for too long, with too little rehabilitation, consuming public budgets and hollowing out African-American communities in particular.

Although the number of people held in state and federal prisons appears to have leveled off at about 1.6 million — 2.2 million if those in local jails are counted — some scholars and activists are calling for far more ambitious change. They ask: Why not reduce the prison population by a quarter or even by half? (That would still leave it far higher than it was a few decades back, when crime was more rampant than today.)

President Obama won wide praise last month when he said to the N.A.A.C.P. that while violent criminals need to be kept behind bars, “Over the last few decades we’ve also locked up more and more nonviolent drug offenders than ever before, for longer than ever before,” adding, “And that is the real reason our prison population is so high.”