By ERICA WERNERWASHINGTON (AP) â¿¿ 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 study from the prominent conservative think tank 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." Former Sen. Jim DeMint, R-S.C., the Heritage Foundation's new president, dismissed such criticism. "It's clear a number of people in Washington who might benefit from an amnesty, as well as a number of people in Congress, do not want to consider the costs," DeMint said. "No sensible thinking person could read this study and conclude that over 50 years that it could possibly have a positive economic impact." The brouhaha developed as both sides prepare for the landmark bill to undergo its first tests later this week in the Senate Judiciary Committee, which will begin voting on amendments Thursday. It underscored the high political stakes for both supporters and opponents, as each jockeyed to define the legislation. And it laid bare splits within the Republican Party, where business-oriented leaders such as Barbour and anti-tax activist Grover Norquist are pushing for immigration reform, while more ideologically focused lawmakers and groups are voicing increasingly loud opposition.