As immigration reform picks up steam in Congress, conventional wisdom holds that a handful of key players are shaping the legislation. Labor unions. Big business. Advocacy groups for and against a path to citizenship for the undocumented. But little scrutiny has been directed at a multi-billion dollar industry with a lot riding on the future of immigration policy: the private companies that operate federal prisons and detention facilities.

For-profit prison management has become a booming business in recent years. Much of that growth is driven by the government’s ramped-up immigration enforcement, which have boosted demand for privately-run prison facilities to detain suspected illegal immigrants until deportation hearings, and to incarcerate immigrants who have been convicted of crimes.

The nation’s two largest private prison operators, the Corrections Corporation of America (CCA) and the GEO Group, have more than doubled their revenues from the immigrant detention business since 2005, contributing to overall combined revenues that eclipsed $3 billion in 2011. Prison companies have spent heavily during this time to influence government: over the last decade, according to The Associated Press, the industry has spent more than $45 million on campaign contributions and lobbying at the state and federal level.

Some of the politicians who have benefited most from this largesse are influential Senators who are now playing key roles in shaping proposed immigration reform legislation.

Among members of Congress, the top two recipients of contributions from CCA are its home-state senators, Lamar Alexander and Bob Corker of Tennessee. The Republican lawmakers, each of whom has received more than $50,000 from CCA according to data compiled by the Sunlight Foundation, represent important swing votes for advancing a reform bill through the Senate. Another top CCA recipient is Arizona Republican John McCain, who has gotten $32,146 from CCA and is a member of the bipartisan “Gang of Eight” that is working to draft legislation. His fellow Gang of Eight member, Marco Rubio, ranks among the top recipients of contributions from the Florida-based GEO Group, receiving $27,300 in donations over the course of his career.

In recent years, each of these senators has sponsored bills that would have increased the detention and incarceration of immigrants. Legislation put forward by Alexander in 2009, for example, would have provided for “increased alien detention facilities.” And a 2011 bill cosponsored by McCain and Rubio sought to expand Operation Streamline, a federal enforcement program that makes illegal entry a criminal offense in some jurisdictions.

The contours of the immigration reform debate are complex, in part because it’s not quite clear yet what “reform” might look like. Various reform proposals might decrease, increase, or have negligible effect on the number of immigrants funneled into the detention system—and thus on the balance sheet of the companies hired by the government to run that system.

But as the immigration reform debate moves forward, reporters would be wise to keep a close eye on how the legislation will affect detention and incarceration levels—and on whether the private prison industry is in fact staying fully on the sidelines, as it insists it does. The key senators who have benefited most from the industry’s donations—Alexander, Corker, McCain, and Rubio—will merit particular attention as they help to shape the bipartisan bill that is considered the most likely blueprint for reform.

Booming industry says it stays out of debate

The explosion of immigrant detention and incarceration is a relatively recent phenomenon. The Immigration Customs and Enforcement budget more than doubled from 2005 to 2012, and now surpasses $2 billion annually. Roughly 400,000 immigrants are now detained each year. ICE’s capacity for daily detention beds has surged from 18,000 in 2003 to 34,000 in 2011—and ICE officials have said they understand the law as requiring the agency to keep these detention beds filled.

Proponents of stronger enforcement say these programs are necessary because immigrants facing deportation can disappear into the shadows if they are not detained, and have little incentive to obey immigration law if there are no consequences for breaking it. “If you’re serious about removing people, then you hold them throughout the proceedings,” said Ira Mehlman, a spokesman for the Federation for American Immigration Reform.

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Sasha Chavkin covers political money and influence for CJR's United States Project, our politics and policy desk. He has written for ProPublica, the Center for Public Integrity, and The New York World. Follow him on Twitter @sashachavkin.