A new study from the conservative Heritage Foundation estimates that granting a path to citizenship for illegal immigrants will cost US taxpayers at least $6.3 trillion.

Heritage Foundation scholar Robert Rector co-authored the long-anticipated study, which is sure to be cited frequently by foes of the immigration reform effort as lawmakers take up legislation to overhaul the nation’s system.

But the study also drew swift criticism from Republicans supporting the reform effort, who called the Heritage Foundation's estimate politicized, exaggerated and flawed in its methodology.

The $6.3 trillion calculation derives from the federal benefits Rector and co-author Jason Richwine believe an estimated 11 million newly legalized immigrants will receive over their lifetimes versus the taxes they will pay.

A summary of the report, for example, states that "former unlawful immigrants together would receive $9.4 trillion in government benefits and services and pay $3.1 trillion in taxes, for a lifetime 'fiscal deficit' - at minimum -- of $6.3 trillion (total benefits minus total taxes.)"

Those benefits, the study states, will eventually include means-tested welfare benefits and health care as well as Social Security payments.

The report's authors acknowledge that their estimated price tag concentrates only on the citizenship piece of proposed immigration reform legislation rather than estimating the costs of the massive bill as a whole. But they argue that the economic benefits of a comprehensive reform that includes a path to citizenship would still be minimal compared to cost of "amnesty."

"No sensible thinking person could read this study and conclude that over 50 years that this could possibly have a positive economic impact," said Heritage president and former senator Jim DeMint at a press conference unveiling the study.

Under the Gang of Eight proposal that was introduced in the Senate last month, qualified undocumented workers could pay fines and back taxes to become eligible to apply for a probationary legal status that -- after 10 years, more fines and a clean criminal record – can be adjusted to legal permanent residency and ultimately citizenship.

During that probationary status, previously undocumented immigrants would not be eligible to receive federal benefits like welfare.

But Rector states that, because the average age of an undocumented immigrant is just 34 years old, the accumulated benefits after these individual become citizens will far outweigh their contributions to the economy.

That’s a calculation that others in conservative community dismiss, including economists like Doug Holtz-Eakin and policy analysts at the Cato Institute who dispute the Heritage Foundation’s methodology and say that the estimate fails to take into account the cumulative effects of immigration reform on America’s economy.

In a conference call sponsored by the Bipartisan Policy Center, former Mississippi governor and onetime RNC head Haley Barbour slammed Heritage's report as a "political document" designed to scare off Republicans inclined to support comprehensive reform.

"That Heritage is trying to kill this in the crib now, I think, is a political statement that they know that this is going to be a movement for reform that’s going to get stronger and stronger because it’s truly good policy," he said.

And Sen. Jeff Flake, a Republican member of the Senate's Gang of Eight, took to Twitter to blast the study shortly after its release.

"Here we go again," he wrote. "New Heritage study claims huge cost for Immigration Reform. Ignores economic benefits. No dynamic scoring."

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