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A group advocating for tighter immigration laws estimates that a hotly debated bill that would give tens of thousands of illegal immigrants who attend college or join the military a path to legal status would cost taxpayers $6.2 billion a year and "crowd out" U.S. students in the classroom.

A new report from the Center for Immigration Studies says the so-called DREAM Act, which would give more than 2 million young immigrants brought to the United States before the age of 16 a chance to become legal residents, could erode the educational opportunities available to U.S. citizens.

The report, "Estimating the Impact of the DREAM Act," comes as Rep. Steve King, R-Iowa, seeks an analysis from the Congressional Budget Office of the legislation's cost before there's a vote. King, who issued a letter Wednesday afternoon to CBO, claims Senate Democrats are hiding an analysis that shows the legislation will cost upwards of $20 billion.

But Homeland Secretary Janet Napolitano said Thursday the CBO scored the current version of the bill in the House as neutral.

"The cost argument really doesn't hold water anymore," she said during a conference call.

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The CIS says its $6.2 billion estimate is conservative and does not include the "modest" number of illegal immigrants expected to attend private institutions. The report assumes that most illegal immigrants would attend state universities and community colleges schools where both funds and slots are limited.

"These institutions where most of these kids can be expected to enroll are in dire straits," Steven Camarota, director of research at the center who authored the report, told FoxNews.com.

He calculated that 1.03 million illegal immigrants will enroll in public institutions and receive a tuition subsidy from taxpayers of nearly $6,000 for each year of attendance for a total cost of $6.2 billion year – a figure that doesn't include other forms of financial assistance that may be provided.

The legislation needs to be funded at his baseline estimate of $6.2 billion each year -- a move that he says Democrats are avoiding because it would destroy support for the bill.

"Right now it's an unfunded mandate that would come at the expense of not just taxpayers but kids trying to get into these schools," he said.

But critics of the group call the report "misleading" and say it lacks evidence supporting its predictions.

Wendy Sefsaf, a spokeswoman for the Immigration Policy Center, said CIS has "a very common formula," which factors the costs of anything without the benefits.

"If you're my roommate, there are costs associated with it," she told FoxNews.com, noting the cost of housing, food and utilities. But she added that the benefits of a roommate paying half the rent, half the utilities and half of the groceries each week should be included in any analysis.

"CIS's cynical mischaracterization of the DREAM Act is not only inaccurate, but hypocritical as well," she said in a statement. "CIS frequently laments that so many immigrants to the United States have low levels of education, yet opposes a measure that would allow some of these immigrants to become more educated."

Her group cited an estimate by the liberal Center for American Progress that it would cost $48.6 billion to deport more than 2 million illegal immigrants who were raised in the U.S.

"The U.S. economy doesn't need more deportations; it needs more college graduates," she said, citing a Georgetown University report that shows Americans in 2018 will fall short of achieving 22 million new college degrees -- the number some say is needed to stay globally competitive -- by at least 3 million.

But Camarota notes that he never said in his report that the legislation is a bad idea; just that it should include funding.

"I said if the act does not want any crowding out effect, then Congress has to provide money to community colleges and state universities," he said.

"The only way there's no crowding out is if you imagine there are not limited spaces, if you imagine there are not limited funds," he said.

He also said in-state tuition subsidies are offered to anyone with legal status, a benefit that wouldn't be eliminated by the DREAM Act because college would otherwise likely be too expensive for most of the beneficiaries.

Senate Majority Leader Harry Reid is trying to muster the 60 votes necessary to move to an up-or-down on the measure in the final weeks of the lame-duck session but Republicans are blocking all legislation until Congress votes on extending the Bush tax cuts for all households and funding for the government.

Reid has infuriated Republicans by pushing four different versions of the bill without a hearing on any of them. The latest version lowers from 35 to 30 the age at which an illegal immigrant would be eligible to go through the program.

Texas Rep. Lamar Smith, the top Republican on the House Judiciary Committee, said he opposes the bill because it offers "mass amnesty," allows illegal immigrants to receive in-state tuition at public universities and is a "magnet for fraud."

"Many illegal immigrants will falsely claim that they arrived in the U.S. before age 16," he said in a written statement. "Once an application is filed, no matter how fraudulent, the federal government is prohibited from deporting the applicant."

"Meanwhile, they will be given work authorization to compete for jobs with the 14.5 percent of Americans under age 35 who are currently unemployed," he said.

Representatives for Reid and Sen. Dick Durbin, D-Ill, the chief sponsor of the DREAM Act, did not return messages seeking comment.

Sefsaf said not passing the DREAM Act is jeopardizing the economic potential of illegal immigrants to buy homes and cars and other things that citizens consume.

But Sefsaf couldn't estimate how much tax or economic revenues would be lost if the legislation falters.

"That's always hard for enumeration," she said.

In his report, Camarota challenges the argument made by supporters of the DREAM Act that the legislation will significantly increase tax revenue because immigrants with a college education will earn more and pay more in taxes over their lifetime.

He says any potential tax benefit is in the long term and won't help public institutions deal with the large influx of new students the act creates in the short term. He also says given the limited spaces at public institutions, "there will almost certainly be some crowding out of U.S. citizens, reducing their lifetime earnings and tax payments."

His report also notes that the DREAM Act only requires two years of college without getting a degree -- a background that only allows modest income gains.