WASHINGTON – A bipartisan Senate immigration bill would cost the government a net $6.3 trillion over the next 50 years to provide benefits for millions of people now living in the U.S. illegally, the Heritage Foundation said in a report Monday, setting off a fierce dispute with fellow conservatives who attacked the study as flawed and political.

The Heritage study said immigrants granted new legal status under the bill would eat up more than $9 trillion in health, education, retirement and other benefits over their lifetime, while contributing only around $3 trillion in taxes. Republicans and conservative groups who support the bill quickly countered that the study failed to measure broader economic benefits from an immigration overhaul, including a more robust workforce that would boost the gross domestic product.

"The Heritage Foundation document is a political document; it's not a very serious analysis," said former Mississippi Gov. Haley Barbour, a Republican who's part of a task force with the nonprofit Bipartisan Policy Center that supports the bill. "This study is designed to try to scare conservative Republicans into thinking the cost here is going to be so gigantic that you can't possibly be for it."