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 senator 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 governor 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 grass-roots 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 wake of last year’s election.

Key conservative stalwarts, including anti-tax 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 (Fla.), 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-Tex.), a tea party favorite who opposes the immigration bill.

“It’s clear a number of people in Washington who might benefit from amnesty, as well as a number of people in Congress, do not want to consider the costs,” DeMint said during a news conference at which he unveiled the Heritage analysis.

On Tuesday, the congressional Joint Economic Committee will open a two-day hearing on the economic consequences of the legislation.

In their analysis, Heritage economists Robert Rector and Jason Richwine argued that illegal immigrants, who could gain citizenship after 13 years under the Senate plan, would pay $3.1 trillion in taxes but receive $9.4 trillion in government benefits in their lifetimes, including Social Security, Medicare, housing and education.

But critics of the Heritage Foundation’s methodology said it failed to account for social mobility among Hispanics. The price tag estimated in the study also includes the costs of educating and providing services to immigrants’ U.S.-born children, but as citizens they are entitled to those benefits regardless of whether the bill passes Congress.

“Here we go again. New Heritage study claims huge cost for Immigration Reform. Ignores economic benefits. No dynamic scoring,” Sen. Jeff Flake (R-Ariz.), a bill co-sponsor, wrote Monday on Twitter.

Douglas Holtz-Eakin, an economist for the American Action Forum, has said that the immigration overhaul proposal could boost gross domestic product growth by a percentage point each year over the next decade. Holtz-Eakin called the Heritage study “a narrow, incomplete look at the immigration reform issue.”

The bipartisan Senate group has pledged that the legislation would be “deficit-neutral,” and members have said that they will amend the bill if the Congressional Budget Office were to ascribe a net cost to it. The senators have emphasized that illegal immigrants will be required to pay at least $2,000 in fees and some back taxes before earning legal status, money that could help pay for other provisions in the bill, such as increases to border security investments.

Aaron Blake and Jim Tankersley contributed to this report.

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