Shares of private prisons GEO Group Inc. and CoreCivic Inc. took a hit Friday morning after presidential candidate Senator Elizabeth Warren tweeted about about her plan to terminate them.

GEO slid as much as 5.8%, its biggest intraday loss since March, and CoreCivic retreated as much as 6%, the largest intraday decline this year.

Warren, a Massachusetts Democrat, said in a tweet that “private prison companies have spent millions to turn our criminal and immigration policies into ones that prioritize making them rich instead of keeping us safe —with terrible consequences. Today I’m announcing my plan to end this private profiteering off of cruelty.”

The plan calls for banning private prisons and detention facilities, stopping contractors from charging service fees for essential services, and expanding oversight. But with Republicans controlling the Senate, Warren will likely hit a wall with her proposal without some GOP support.

Private prisons have been targeted by Warren in the past, when the senator said she was looking into the system’s approval process, saying the group accrediting operators appears to be a “conflicted party” that doesn’t hold facilities accountable.