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.

The government programs behind this surge in detentions include initiatives known as 287(g) and Secure Communities, which authorize state and local law enforcement, and local jails and prisons, respectively, to look up the immigration status of suspected criminals. Individuals who are found to be undocumented may be placed into deportation cases and potentially detained. Another important program is Operation Streamline, which increased the penalties for illegal border crossers, with the result that many serve time in federal prisons for designated immigration offenders. Some of these prisons are operated by CCA and GEO Group.

And as is usually the case for companies that do big business with the government, the private prison industry is active on the lobbying front. Records from the Lobbying Disclosure Act database show that both CCA and GEO Group have regularly lobbied the House and Senate, as well as executive-branch agencies, on immigration-related matters in recent years. But both companies say that while they lobby the government to obtain contracts, they never seek to shape the policy that determines who is detained.

“It is CCA’s longstanding policy not to draft, lobby for or in any way promote detention enforcement legislation,” CCA spokesman Steve Owen said in an email. “That means CCA does not take a position on or advocate for or against any specific immigration reform legislation nor does our government relations team on our behalf.”

GEO Group spokesman Pablo E. Paez replied in an email that “the GEO Group has never directly or indirectly lobbied or advocated to influence immigration policy.”

These claims of a hands-off approach to immigration policy debates have been called into question in the past, however. A 2010 investigation by NPR found that CCA had participated in a “quiet, behind-the-scenes effort to help draft and pass” the controversial Arizona law that allowed police to demand papers from individuals who were suspected of being undocumented immigrants and lock up those who failed to provide them. Most of that law has since been struck down by the Supreme Court.

When asked whether CCA had lobbied Sen. Alexander regarding immigration enforcement or reform, Alexander spokesman Jim Jeffries replied in an email: “Corrections Corporation of America is an important Tennessee company. Sen. Alexander and his staff have hundreds of conversations a week with his constituents and he believes they are entitled not to have those conversations publicly reported.” (Throughout his career, Alexander has enjoyed close ties to CCA: his former special assistant, Charles L. Overby, sits on CCA’s board. And his wife, Honey, made an early $5,000 investment in CCA in the first part of a transaction that later came under scrutiny.)

In reply to an inquiry about whether Sen. McCain had discussed either Operation Streamline or pending immigration reform proposals with CCA, McCain spokesman Brian Rogers replied that the enforcement program “will continue whether or not Congress passes comprehensive immigration reform. Senator McCain supports Operation Streamline because it works.”

A spokesperson for Sen. Corker said he was traveling overseas and unable to respond. Sen. Rubio’s office did not respond to email and telephone inquiries.

Impact of reform unclear

While the companies insist that they do not seek to shape immigration policy, the private prison industry has at times acknowledged its business could be affected by the reform debate. “Immigration reform laws which are currently a focus for legislators and politicians at the federal, state and local level also could materially adversely impact us,” the GEO Group declared in a 2011 SEC filing. And with the White House saying its goals for reform include “expanding alternatives to detention and reducing overall detention costs,” the limited media coverage so far has focused on potential losses for the industry. “Private prisons will get totally slammed by immigration reform,” read the headline on a Feb. 2 Business Insider piece.

But any immigration reform bill will be shaped by Congress—and the impact of reform on detention and incarceration still hangs in the balance. Divisions have already emerged between the White House and the Senate’s bipartisan Gang of Eight about the crucial path to citizenship. One key difference is that the Gang of Eight—which includes McCain and Rubio—has proposed that the Homeland Security department must certify that the border is secure before any undocumented immigrants can get green cards.

One reporter, Seth Freed Wessler of the progressive media site Colorlines.com, has suggested that as negotiations unfold, reform might even increase the numbers of immigrants being imprisoned. A concern among Democratic staffers and immigrant advocates, Wessler wrote, is that Republicans may insist on more enforcement—such as, perhaps, an expansion of Operation Streamline—in exchange for agreeing to a path to citizenship.

Provisions to expand the population facing mandatory detention and deportation—for example, adding categories such as suspected gang members—are another means by which comprehensive reform might lead to more immigrants being detained, Wessler said in an interview.

Muzaffar Chishti, director of the Migration Policy Institute’s office at New York University, said he thought it was unlikely that legislators would seek to exert much influence over executive-branch enforcement programs such as Operation Streamline and Secure Communities.

But, Chishti said, “McCain will want to stay relevant in the Gang of Eight, so he’ll want to put his preferred items in the enforcement agenda.” And in general, lawmakers “could start introducing elements in the legislation saying these are triggers that indicate the borders are secure.”

CCA, for its part, has said it anticipates continued strong demand from the government, regardless of whether a reform bill is passed. A subsequent article by Wessler quoted CCA President and CEO Damon Hininger telling investors last week that while the profile of ICE detainees may change over time, “I think their general belief is there’s always going to be a demand for beds.”

But the murkiness of reform’s impact on the industry is one of the features that makes this such a compelling—and challenging—story. In the coming weeks and months, reporters will be tracking the flurry of competing reform proposals coming from lawmakers, and the ways that those proposals reflect the priorities of competing interest groups. As the story of immigration reform gets told, let’s not forget about an industry that has more than a few dollars at stake.

Jose Robledo contributed reporting for this story.

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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.