By Caren Bohan
WASHINGTON, June 13 (Reuters) - As the U.S. Senate debates the most far reaching immigration bill in a generation, all eyes are on Cuban-American Senator Marco Rubio, wondering if he might walk away from it.
There are concerns that the rising political star from Florida, crucial to attracting Republican support for the legislation, may decide it doesn't do enough to bolster security on the U.S.-Mexico border - a priority for conservatives.
But another Cuban-American, Senator Robert Menendez, could be equally important to the bill's fate, even though the Democrat from New Jersey has kept a lower profile than Rubio as one of the eight Senate co-authors of the immigration bill.
Menendez' concern - the opposite of Rubio's - is that the bill could become so tough as to gut its centerpiece, a path to citizenship for millions of undocumented immigrants.
The only Latino Democrat in the Senate, Menendez has the trust of immigrant groups that have been critical to the bill's momentum. If he were to walk away, so might many Latino backers of the bill, along with other Democratic lawmakers.
"I don't envision" withdrawing support, Menendez said. But he would "reserve the right," he told Reuters. "What I am not going to sell to the (Latino) community is something that does not meet our collective core principles."
Menendez' words are taken seriously. He pulled his support from an immigration bill in 2007 when he thought it broke with his principles and the interests of the Latino community.
Rubio and Menendez, along with six other Republicans and Democrats, form the bi-partisan Gang of Eight, which is steering the legislation's passage. The possibility of a gang implosion hangs over the debate now underway in the Senate.
The bill that the gang rolled out in April would provide a 13-year path to citizenship for 11 million undocumented immigrants.
But to garner Republican votes, it exacts a price: it toughens border security and imposes a series of burdens on undocumented immigrants, including fees and fines.
If the Senate makes the pathway too easy, it would lose Republicans and its prospects would become even more challenging in the Republican-led House of Representatives. But if the bill makes the path too arduous, it could lose Menendez and Democrats, who control the U.S. Senate.
PUSH AND SHOVE
The push-and-shove has begun, and Rubio and Menendez are in the middle of it.
Rubio, the Florida Republican, has been in talks with Texas Republican Senator John Cornyn over an amendment Cornyn is sponsoring that would delay green cards for undocumented immigrants if certain border-security targets are not met. Rubio is "supportive of the idea," said his spokesman Alex Conant.
But Menendez cited the Cornyn amendment as exactly the type of change to the bill he will fight, saying it would undermine the pathway to citizenship "because it creates triggers that can never be achieved."
The differences between Rubio and Menendez don't stop there.
Rubio, 42, was elected to the Senate in 2010 and is a novice in the Congress. The 59-year-old Menendez is a chairman of the Senate Foreign Relations Committee and a 20-year veteran of Congress, starting as a House member from New Jersey and then moving to the Senate in 2006.
Rubio is a media-magnet, thanks to his charisma, his 2016 presidential possibilities and his unique role as a Cuban-American senator in a party trying to reach out to Hispanic voters.
Menendez has kept a lower profile, although he does give frequent interviews to Spanish-language media.
While Rubio was being toasted last year as the new face of the Republican Party, Menendez was fighting off allegations about his relationship with Salomon Melgen, a political donor and friend.
Immigration advocates believe the Melgen controversy may be part of why Menendez has shown a preference for a more low-key public profile.
Menendez disagrees. Working out of the limelight, he said, is his nature. "I consider myself a workhorse versus a show horse ... I don't need to try to be a press hound for the purposes of getting credit."
He is more of a "behind-the-scenes guy," said Ross Baker, a political science professor at Rutgers University in New Jersey. "He's very cerebral and a cool customer."
He was part of Senate group that drafted a pathway-to-citizenship proposal in 2007 but ended up walking away from it when, at the urging of Republicans, the measure was changed to sharply reduce the role of family ties in legal immigration.
"He plays a very important role in the sense that if those things - the legalization and citizenship piece - are put under threat, his voice is going to reverberate all over the Senate on that," said Clarissa Martinez-De-Castro, director of civic engagement and immigration at the National Council of La Raza, the largest U.S. Hispanic civil rights group.
Martinez-De-Castro praised Rubio for supporting the immigration bill but said the Republican senator is not the one "carrying the water" on issues such as the path to citizenship.
"Rubio represents where the Republican party wants to go. And I think that Menendez represents the Latino community writ large," she said.
Mee Moua, the head of the Asian American Justice Center, sees Menendez as an "inside champion," especially on issues of family unification raised by the bill.
Family unification remains a priority for Menendez as the Senate debates immigration.
Many Republicans believe the current immigration system places too much weight on family ties in the awarding of green cards and want to see more emphasis on the work skills of people seeking to come to the United States.
But Democrats argue that preserving family unity should be an important goal of the immigration system.
During the Gang of Eight's closed-door negotiations, Menendez pressed Republicans such as South Carolina Senator Lindsey Graham to preserve some emphasis on family ties. Under a compromise, the sibling category for green cards was eliminated. Sponsorship of adult children was preserved but with new limits. Unmarried children could get green cards and married sons and daughters 31 and under would also be eligible.
"The two driving goals are that there must be a legitimate and achievable pathway to citizenship and that we have to preserve the core family reunification as a principle of our immigration reform," Menendez said.
Scott Corley, executive director of Compete America, said he did not think the other members of the gang would allow changes to the immigration bill that Menendez could not support.
"I don't think they will even consider anything that causes Bob Menendez to walk. I think (Republican Senator) John McCain would walk with Bob Menendez if something like that were to happen," he said. McCain, a former Republican presidential candidate, is also a member of the Gang of Eight. (Reporting By Caren Bohan; Editing by Fred Barbash, Martin Howell and Tim Dobbyn)