The GEO Group—a shady, for-profit prison company—has invested more heavily in Marco Rubio for years and his presidential campaign is no exception.

This past spring, the organization gave Rubio super PAC Conservative Solutions a fat check for $100,000, a massive donation that dwarfs the prison organization’s prior contributions to the freshman senator.

The donation also ups the ante of already well-established support for Rubio, empowering the Rubio super PAC to boost the senator’s candidacy for the White House.

When challenged about his donors by the press or on the campaign trail, Rubio often says that contributors are buying into his agenda, not the other way around. But since the failure of the Gang of Eight immigration reform package, it is worth noting that Rubio has to an enforcement-first, border security approach—which, consciously or not, would benefit private prison organizations like GEO Group.

Rubio isn’t the only presidential candidate to benefit from the GEO Group’s political largess: The organization also gave $100,000 to Jeb Bush’s super PAC, for example, around the time of the Rubio contribution. Bush wrote in the mid-’90s about the need for more for-profit prisons.

But Rubio’s close ties with the prison company are unique, and GEO’s historical support for Rubio is substantially greater than that for Bush.

Before the most recent six-figure contribution, Rubio had received nearly $40,000 in campaign funds from GEO, making him the number one Senate recipient of the group’s political donations. And Rubio’s first Senate chief of staff, Cesar Conda, is a founder of what has become GEO’s main lobbying firm, Navigators Global. Navigators Global’s employees have also been frequent contributors to Rubio’s political efforts.

The Rubio campaign told The Daily Beast that it didn’t comment on outside groups. A spokesman for Conservative Solutions declined to comment.

The GEO Group did well when Rubio was speaker, having been awarded a $110 million state government contract right after Rubio hired a consultant who had been a trustee for a GEO real estate trust.

Rubio voluntarily disclosed in 2008 that he had raised $50,000 from GEO Group for organizations that he was involved with.

The private prison company is naturally controversial since the Florida-based group makes its money in part by detaining undocumented immigrants—in total, it runs 64 prison facilities in the United States, with a total of 71,973 beds.

If the federal government further enhances border enforcement, as many Republicans are calling for it to do, it stands to profit from holding foreigners who have violated the law.

The GEO Group, the nation’s second-largest private prison company, holds as its largest client the Federal Bureau of Prisons. And business has been good: Between 2000 and 2012, net income jumped from $16.9 million to $78.6 million, in part due to federal government efforts to charge those who cross the border illegally.

The GEO Group has lobbied to increase the number of mandatory immigration detention spots, having already benefited from Congress’ requirement that the feds maintain approximately 34,000 detention beds.

They’ve been accused of mistreating undocumented immigrants, including providing insufficient medical care and even allegations of sexual assault against its guards (a subsequent Department of Homeland Security investigation found no evidence to support the assault allegations).

And as The Daily Beast reported, GEO Group is dealing with a lawsuit from former detainees, who have accused the prison group of engaging in human trafficking to increase its profits.

The GEO Group, for its part, said that it does not “take a position or advocate for any specific criminal justice or immigration policy,” but that like many other corporations “participates in the political process.”

“Our company’s political and lobbying activities focus entirely on promoting the use of public-private partnerships across correctional and detention services and in the delivery of offender rehabilitation and community reentry programs, and they do not entail any advocacy either for or against criminal justice or immigration policies,” a company spokesman said. “Over the last 30 years, our company has made significant investments in the development of evidence-based offender rehabilitation and community reentry programs aimed at reducing recidivism and helping the men and women in our care reintegrate into society.”