SPRINGFIELD — Republicans aren’t alone in their criticism of a plan to convert a state prison to house terrorist detainees.

The state’s largest employee union said Tuesday that Gov. Pat Quinn needs to rethink selling the mostly vacant Thomson Correctional Center to the federal government.

“The governor’s rush to sell Thomson ignores serious questions about the dangerously overcrowded, understaffed Illinois prison system,” said Henry Bayer, executive director of the American Federation of State, County and Municipal Employees union in Illinois. “How can state prisons function safely with more than 45,000 inmates jammed into facilities built for 32,000?”

The union, which represents prison guards and other Illinois Department of Corrections workers, has fought with Quinn over the governor’s plan to lay off employees as a way to help balance the budget. That issue currently is tied up in court proceedings in southern Illinois.

AFSCME also opposes Quinn’s proposal to alter state worker pensions to save money.

On Tuesday afternoon, Quinn is set to meet with President Obama to discuss the sale of the $140 million lockup.

Bayer said events this week regarding the Department of Corrections underscore the union’s concerns selling off the 1,600-cell maximum-security facility at a time when the agency is understaffed.

For example, a prisoner was shot and killed after he held a prison worker hostage at the Pinckneyville Correctional Center.

“Just yesterday, an employee was held hostage for hours by an inmate at a southern Illinois prison, a standoff that ended with the inmate shot to death. The day before, news reports revealed a secret plan that let hundreds of inmates, including violent offenders, walk free after serving just days of their court-ordered sentences,” Bayer said.