Former Secretary of State Hillary Rodham Clinton, Senator Ted Cruz and Senator Rand Paul want to ease mandatory minimum sentences. Gov. Chris Christie wants to release nonviolent offenders pending trial without bail. Gov. Scott Walker, former Gov. Rick Perry and former Senator James Webb want to expand drug treatment as an alternative to prison. Senator Marco Rubio wants to make it harder to convict federal defendants without proving intent.

The focus on overhauling the criminal justice system comes at a time of protests over the use of lethal force by the local police and unrest in cities like Baltimore and Ferguson, Mo., and represents a profound shift in American politics. Where the elder George Bush won the presidency in part by attacking his opponent as soft on crime and Bill Clinton enacted landmark crime legislation pouring police officers into the streets and ratcheting up sentences, today’s candidates across the ideological spectrum have concluded that previous leaders went too far.

“This really does reflect a huge change in the political momentum from decades when parties and candidates competed to see who could be the most flamboyantly punitive,” said Michael Waldman, the president of the Brennan Center for Justice at New York University’s School of Law and a former aide to Mr. Clinton. Now, Mr. Waldman said, “there’s a competition for reform and to take on the issue of mass incarceration. It’s really unheard-of in recent decades.”