State and federal prisons held an estimated 1.56 million inmates at the end of last year, according to the Bureau of Justice Statistics. About 19 and 7 percent of federal and state prisoners, respectively, were held in private facilities.

Shares of prison managers Corrections Corporation of America and Geo Group slid following the tweets.

In a series of tweets, Clinton said the United States needs to end private prisons and mass incarceration. Clinton also criticized punishment for drug offenses and other aspects of criminal justice that she said disproportionately affect African-Americans.

Clinton and other politicians have criticized the rate of incarceration in the United States, particularly among African-American men. Her nearest rival for the Democratic nomination, Sen. Bernie Sanders of Vermont, introduced a bill last month that would ban private prisons, saying they incentivize keeping low-level offenders locked up.



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In a statement, CCA spokesman Steven Owen touted the benefits of private facilities, saying politicians criticize them "without themselves providing any solutions to the serious challenges our corrections and detention systems face."

"Overcrowding, high recidivism rates and skyrocketing costs aren't solved with politics or posturing. They're solved with the hard work that chaplains, teachers, principals, counselors and correctional officers like ours put in every day," he said.

Geo Group told CNBC it is accountable to the government and taxpayers and upholds quality standards. The company added that it has "worked to deliver education and rehabilitation programs and post-release, re-entry services to thousands of inmates."