WASHINGTON - Leading conservatives engaged in a bitter public fight Monday over the costs of overhauling the nation's immigration system, exposing a rift within the Republican Party days before the Senate is set to begin debating a comprehensive reform proposal.

The Heritage Foundation, led by former GOP Sen. Jim DeMint, released a study Monday that estimated that a bipartisan immigration proposal being considered in the Senate would cost U.S. taxpayers $6.3 trillion in coming decades, mostly because of health-care and social service costs for 11 million illegal immigrants who could become citizens.

But the analysis - the estimate in which represented a dramatic increase over the $2.6 trillion tab that Heritage calculated in a similar study six years ago - was quickly denounced by other conservatives, who argued that the think tank purposely overlooked the role that immigrant workers could play in helping the economy grow.

"The Heritage Foundation document is a political document. It's not very serious analysis," said former Mississippi Gov. Haley Barbour, a Republican who is involved in a bipartisan group that supports immigration law reform. "The study is designed to try to scare conservative Republicans into thinking the costs will be so gigantic you can't possibly be for it."

The dispute could go a long way in determining the future of the sweeping immigration proposal, which faces fierce resistance from many Republicans who oppose allowing workers who entered the country illegally to become legal residents.

As the Senate Judiciary Committee prepares to begin considering amendments to the bill Thursday, opponents have accelerated a campaign to discredit broad portions of it. The Tea Party Patriots, a conservative grassroots group, launched a campaign Monday called "No More Train Wrecks," aimed at convincing lawmakers that immigration reform is too costly by comparing it to President Obama's 2010 health-care law.

"Our nation simply cannot afford to provide every benefit of citizenship - including unlimited access to our nation's welfare and entitlement programs - to millions of illegal immigrants," Sen. Jeff Sessions (R., Ala.) said in a statement lauding the Heritage report.

Conservative opposition helped sink a comprehensive immigration bill in 2007, but advocates contend that the politics have changed because Republicans are eager to expand their appeal to Latinos and Asian Americans in the aftermath of last year's election.

Key conservative stalwarts, including antitax advocate Grover Norquist, the Cato Institute and the American Action Forum, have come out in favor of immigration reform, citing the potential for economic growth. Sen. Marco Rubio of Florida, a tea party favorite, is one of four Republicans who authored the proposal with four Democrats.

But DeMint, who retired in December as a senator representing South Carolina, is considered an influential voice among conservatives and has been a longtime mentor to Rubio and Sen. Ted Cruz (R., Texas), a tea party favorite who opposes the immigration bill.