* Senate Judiciary debate gets under way on Thursday
* Special interests to push controversial amendments
* Supporters drive for lopsided Senate approval
By Caren Bohan and Thomas Ferraro
WASHINGTON, May 6 (Reuters) - The U.S. Congress this week opens its first debate in six years on a comprehensive immigration reform bill, testing whether business and labor groups can hold together on a delicately crafted deal that already is under attack.
For 11 million illegal residents, the legislation in the U.S. Senate is their best hope of removing the threat of deportation and charting a path to eventual U.S. citizenship after a major push in Congress to reform a 1986 law died in 2007.
The ambitious bill would put more federal dollars into strengthening the southwestern U.S. border against illegal crossings and aims to revamp a dated visa system so that more foreign workers - high- and low-skilled - could enter.
But in a preview of the tough fight ahead, the conservative Heritage Foundation on Monday released a study concluding that the legislation would end up costing the U.S. government $6.3 trillion over the next 50 years as illegal immigrants become citizens and thus eligible for government programs.
The study was quickly attacked by other conservatives, who said it failed to take into account the economic benefits of legalizing the 11 million people.
Amendments to the 844-page bipartisan bill, the product of months of negotiations, were flowing into the Senate Judiciary Committee, which will begin considering the measure on Thursday.
If its backers including President Barack Obama get their way, the legislation will emerge from the committee later this month positioned for approval by an overwhelming majority in the full Senate.
But first, groups ranging from gay rights activists to construction industry representatives are converging on Capitol Hill, trying to win changes.
“Every lobbyist who has any interest in immigration reform is going to be all over the Hill,” said Emily Lam, of the Silicon Valley Leadership Group, which represents high-tech companies such as eBay, Microsoft and Yahoo! as well as other companies such as Citibank and Verizon.
In the first quarter of 2013, 500 organizations and companies registered to lobby on immigration. A sampling includes the Commissioner of Baseball, MGM Resorts, the U.S. Olympic Committee, Perdue Farms, JPMorgan Chase & Co., and a variety of companies in the business of secure identification, such as Cogent, a 3M Company .
The lobbying activity has the eight senators who wrote the bill - four Democrats and four Republicans - nervous that any change could unravel the entire effort.
But that is not discouraging lobbyists from trying.
The construction industry and other business representatives plan to push for increases in the number of low-skilled foreign workers they would be allowed to hire. If successful at any stage of a complicated legislative process, organized labor’s support for the bill could erode amid fears the bill would undercut American workers.
Several business groups also are concerned about E-Verify, a system for checking the legal status of workers. All businesses would be required to use it for new hires. Businesses do not object to the mandate but want to make sure they are not held liable if the system turns up erroneous information.
Meanwhile, gay-rights activists want the legislation to allow U.S. citizens and permanent legal residents to sponsor their same-sex partners for residency.
Senate Judiciary Committee Chairman Patrick Leahy of Vermont introduced the same-sex partner provision as a separate measure earlier this year, along with Republican Senator Susan Collins of Maine. Immigrants’ rights groups expect him to offer it as an amendment to the immigration bill.
Including such an amendment “will virtually guarantee that it (the bill) won’t pass” the Senate, said Senator Marco Rubio of Florida, one of the eight senators who wrote the legislation.
He made the prediction in an interview last week with the newspaper Politico.
If any of the controversial amendments do clear the Senate Judiciary Committee, they could complicate strategists’ efforts for a resounding vote in the full 100-member Senate.
Anything less than 70 votes for passage, they fear, may not be enough to build the type of momentum needed to get the measure through the more resistant Republican-led House of Representatives.
“Seventy (Senate) votes would give the bill a big boost,” a senior Senate aide said. “Less than 70 wouldn’t kill it, but it would make it a heavier lift.”
The strategy of building a big Senate vote to help create momentum to get legislation through the House worked twice before this year: Once on a bill to avert “the fiscal cliff” of steep, across-the-board tax hikes, and also on a measure to renew a landmark law combating domestic violence against women.
The Senate passed the two measures on bipartisan votes of 89-8 and 78-22, respectively.
In both cases, House Speaker John Boehner cleared the way for passage even though a majority of his Republican lawmakers were not on board.
House Speakers generally do not like passing bills opposed by the members of their own party and if they do it too often, it could stir up a political firestorm.
But with the growing number of Hispanic-Americans becoming more influential in U.S. elections, Boehner has expressed support for immigration reform, though he has not endorsed the Senate bill.
“Boehner could decide to save Republicans from themselves by allowing immigration reform to pass,” said Ron Bonjean, a former House Republican leadership aide turned political strategist. “And it could cost him the speakership,” he added.