Immigrant advocates are battling private prison interests by pushing for wider use of these other options. The question is whether those alternatives will be adopted. The government now spends more than $2 billion a year on immigration detention, while spending only $72 million on alternatives to detention.

"Between 2007 and 2009, when earnings for the S&P dropped by 28 percent, ours grew by 18 percent," said Damon Hininger, CEO of Corrections Corporation of America, during a conference call with investors in May 2010.

So far this year, CCA has spent nearly $1 million on lobbying, according to government disclosure records. The company is publicly traded on the New York Stock Exchange and is worth $4 billion. Private prisons say their lobbying efforts are aimed at promoting their services, not shaping immigration policy.

"We've worked in partnership with the Immigration and Customs Enforcement...for 30 years to help safely and humanely house detainees at a savings to taxpayers, and we will continue to work with them in whatever capacity they need," CCA spokesman Steve Owen said in an email.

But immigrant advocates say the private prison industry is always lobbying for more detention beds. And the consequences of government spending on prison beds can be profound, said Bob Libal, director of Grassroots Leadership, an immigration advocacy group.

Congress has appropriated funds to pay for housing 34,000 illegal immigrants a day, and ICE officials "interpret that as a mandate to fill those beds regardless of what the situation outside is," said Libal. "It keeps people in detention and helps [companies'] bottom line, because half of those 34,000 beds are operated by private prison corporations."

At present, prisoners serving a sentence for an immigration offense in the federal prison system are one of the nation's fastest-growing prison populations, according to data from the Bureau of Prisons. Meanwhile, just about every state's prison costs go over budget. In 2010 New York, Illinois, Texas, Connecticut, Pennsylvania and Missouri were more than 20 percent over their corrections budgets, a study by the Vera Institute of Justice found.

Several pending immigration bills would increase the number of incarcerated immigrants even more.

The Corker-Hoeven Amendment in the Senate immigration bill - the one approved in June - would dramatically increase enforcement at the border, upping border spending by $45 million a year, and thus indirectly leading to more incarcerations. The Senate bill would also convert more immigrants from the civil system to the criminal system, which means they would be held longer.

The House has five pending immigration-related bills of its own. One, called the SAFE Act (Strengthen and Fortify Enforcement), would make it a federal crime for illegal immigrants to be in the U.S., and calls for mandatory detention if local police suspect someone might be undocumented.