Sen. Bernie Sanders vowed during the latest Democratic debate to dramatically reduce America’s prison population in his first four years in office.

“Here’s my promise: At the end of my first term as president we will not have more people in jail than any other country,” Sanders said. “We will invest in education, and jobs for our kids, not incarceration and more jails.”

The U.S. imprisons about 2.2 million people, making the country the world’s biggest jailer by far. China, which comes in second, jails 1.7 million people.

It’s unclear how Sanders plans to empty American prisons by half a million people, since the president and federal government preside over only a fraction of the total prison population. Fewer than 200,000 of the nation’s prisoners are in the federal system; the vast majority live in state and local jails.

Sanders has called for an end to putting drug users in jail, but removing non-violent drug users from the nation’s jails would still not reduce the total population by half a million people.

Sanders’ promise came during a back and forth with Hillary Clinton over their plans to reform the criminal justice system, an issue Sanders has been more vocal about as he also works to win over minority voters. The candidates were asked how they would address high incarceration rates, which in Wisconsin reach a stunning 13 percent for black males. Sanders called the high rate of incarceration “unspeakable,” and vowed to stop punishing people for using marijuana. Clinton, however, delivered a far more detailed answer, mentioning Dontre Hamilton, a young man who was shot and killed by a police officer in Milwaukee who was not charged in the incident. She also mentioned that so many criminal-justice policies are made at the local level.