A bipartisan group of eight senators, the so-called Gang of Eight, will unveil their proposed immigration reform bill on Tuesday, after finishing up the final details of the legislation that would provide a pathway to citizenship for the 11 million people living illegally in the U.S.

Even before the measure gets its first public airing, its authors were defending the program.

"I'm not convinced. I know Senator Rubio's heart is exactly right. And I really respect the work of the 'Gang of Eight.' But they have produced legislation ... that will give amnesty now, legalize everyone that's here effectively today and then there's a promise of enforcement in the future."

- Sen. Jeff Sessions, R-Ala.

"We're not awarding anybody anything. All we're doing is giving people the opportunity to eventually earn access to our new, improved and modernized legal immigration system," said Sen. Marco Rubio, a Florida Republican who has been among the lawmakers at work on the overhaul.

Rubio had been slow to fully embrace the proposals that strengthen border security while offering a long pathway for those immigrants to earn citizenship. But he promoted the pending legislation to seven Sunday show hosts, leaving little doubt he was going to work to win over skeptics.

"What we're working on is a starting point. It is not the take-it-or-leave-it offer. It is a starting point of reform," Rubio said.

But the proposal has its critics too.

"I'm not convinced," said Sen. Jeff Sessions, R-Ala. "I know Senator Rubio's heart is exactly right. And I really respect the work of the 'Gang of Eight.' But they have produced legislation ... that will give amnesty now, legalize everyone that's here effectively today and then there's a promise of enforcement in the future."

Added Sen. Mike Lee, a Utah Republican: "The pathway to citizenship, right now, before those other elements are in place, is the deal-breaker for me."

Lee said he could consider supporting the proposals only if the first priority were border security.

It is, Rubio insisted.

"If you are undocumented here now, if you are illegally in the U.S., that you can't even apply for this until these plans are in place and they begin to implement them," Rubio said. "And then you're going to have to pay a fine. You're going to have to pay an application fee. You're going to have to pass a background check."

Other lawmakers helping write the legislation acknowledged the political challenges of the issue, even with the tough measures that could take more than a decade for those here illegally to overcome.

"A lot of my conservative colleagues have significant questions and they're legitimate," said Sen. John McCain, R-Ariz. "This is the start of a process, this is a vehicle that requires hearings, requires input and we welcome all of that. ... I am guardedly optimistic that we will see finally the end of this long, long trek that a lot of us have been on for many years."

The measure would put millions who are in the U.S. illegally on a 13-year path to citizenship, while toughening border security requirements, mandating that all employers check the legal status of workers, and allowing tens of thousands of new high- and low-skilled workers into the country with new visa programs.

But Rubio also left himself a way out if compromises yield an unsatisfactory bill. With an eye on a possible White House run in 2016, Rubio, a tea party favorite, has been careful not to appear weak on border security or create political problems among the conservatives who have great sway in picking the GOP's nominee.

"I've been very clear about my principles of what reform needs to look like. And if this bill were ... somehow to abandon those principles via the amendment process or what have you, certainly I wouldn't support that," he said.

Rubio pressed his case during interviews on ABC's "This Week," CBS' "Face the Nation," CNN's "State of the Union", "Fox News Sunday" and NBC's "Meet the Press." He also was booked on the Spanish-language networks Telemundo and Univision. Sessions was on ABC. Lee was on NBC. McCain appeared on CNN.

Based on reporting by The Associated Press.

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