“This is a conversation that is unrecognizable from 10 years ago — even five years ago — when these kinds of proposals wouldn’t have been floated in back rooms, let alone in public,” said Adam Gelb , president of the Council on Criminal Justice , a nonpartisan research organization.

This week, the leading progressives in the Democratic field elevated some of those ideas in the presidential race.

Senator Elizabeth Warren of Massachusetts became the latest Democratic contender to release a criminal justice plan on Tuesday, and like other candidates, she called for releasing more people from prison early, eliminating private prisons and strengthening oversight of the police.

Two days earlier, Senator Bernie Sanders of Vermont introduced his own plan, which in some respects went even further. He called for banning the use of facial recognition software by police departments, raising the age of adult criminal liability to 18 and allowing certain areas to be set aside where people could legally inject intravenous drugs.

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Criminal justice is just one of the areas in which the Democratic presidential candidates have moved leftward in the proposals they are advocating in the primary race. In the policy debate over subjects like immigration, health care and gun control, the shift has been unmistakable.