Some activists, however, say Rubio has a "fatal flaw" embedded in his immigration policy that ensures reform will never happen if he makes it to the White House.
Frank Sharry is the founder and executive director of America's Voice, a group that was founded to serve as the communications hub of the immigration movement. He practically seethed when Business Insider asked him about Rubio's immigration plan.
"It's unf---ingbelievable to me that this hasn't been exposed," Sharry said. He later added, "He's usually mentioned in the same sentence with Jeb Bush, and there is no relationship to the reality."
Rubio was previously seen as a staunch ally of the immigration-reform movement. In 2013, he was one of the four Republicans in the bipartisan Gang of Eight who crafted a comprehensive immigration-reform bill in the Senate. Rubio has reportedly highlighted his work on that legislation to curry favor with pro-business donors who support immigration reform.
The Gang of Eight passed the Senate, but the Republican-led House never took up the legislation.
Rubio has since abandoned comprehensive immigration-reform legislation for what he describes as a more realistic approach. His current plan calls for dealing with border security and illegal hiring before other components of reform - changing the green card and visa system, or addressing the approximately 11 million immigrants who don't have permission to be in the US.
Alex Conant, the communications director for Rubio's campaign, described this shift when Business Insider asked him about the criticism from immigration-reform activists. He said the failure of the Gang of Eight bill showed the need for a different strategy.
"The all-or-nothing approach will continue to leave immigration-reform advocates with nothing," Conant said. "The only way to fix our broken immigration system is to first secure our borders."
Sharry dismisses this as "campaign-oriented tortured doublespeak." He said it is designed to help Rubio maintain support from pro-reform members of the business community and Latino voters without angering the conservative base in the crowded GOP primary.
"He wants people to think he's for reform when, in fact, his approach means no reform ever," Sharry said. "He's doing this so he can say to donors and to Latinos, 'I'm with you,' and say to conservatives who are angry at him for working on the Gang of Eight bill, 'I'm with you.' ... The right loves it, because they know that immigration reform will never happen under his plan."
Rubio most recently outlined his immigration policy at the Voter's First Forum in New Hampshire on Monday, when he said there was "only one way forward" on immigration reform.
"It would require three steps, and they have to happen in the following sequence," Rubio said. "First, we have to prove to the American people that illegal immigration is under control. It's not good enough to just say we're going to pass a law that will bring it under control. People demand to see it."
"They want to see the fence completed, they want to see more border agents, they want to see more drones and cameras, and ground sensors," Rubio said. "But they also recognize that over 40% of the people in this country illegally entered legally and overstayed a visa.
"That's why we need an electronic verification system that employers must comply with or they will be heavily fined," he added. "And that's why we need an entry/exit biometric system at our seaports and airports so that we know when people are overstaying visas and we can identify them. That is the key that unlocks the ability to make progress on anything else when it comes to immigration."
After this first step, Rubio said he would "modernize our legal immigration system" so it is based on "merit" rather than whether someone has family members living in the country. The final step of his plan, he said, would be finding "a reasonable way to address the fact that you have 12 million people living in this country or more who are illegally here but have been here for a long time."
"They will have to pass a background check, they will have to pay a fine, they will have to start paying taxes, they will have to learn English," Rubio said of immigrants who lack proper documentation. "In exchange for that, what they will get is a work permit that allows them to legally work in the United States and travel, and that's all they will have for an extended period of time. And then at some point in the future, we can have a further conversation about whether they're allowed to apply for a green card."
"His major point is that once we prove that we've secured the border and that we've eliminated illegal hiring, then we should deal humanely with the 11 million people here, and I prefer citizenship not just a permanent block on citizenship," Sharry said of Rubio's plan.
Though Sharry said Rubio's stated openness to a pathway to citizenship for immigrants who lack documentation "arguably puts him in better shape than Jeb," he added that this difference was meaningless because of the three-step sequence outlined by the senator. Sharry said the security measures Rubio set as a barrier to immigration reform would be unable to pass in Congress.
"That's like saying, 'As soon as I gorge on food this week, my diet is going to be so good,'" Sharry said. "The idea that we're going to pass legislation ... the idea that he would get Democrats in the immigration-reform movement to support doing all the things Republicans want in exchange for down the road maybe doing something for the 11 million? ... I don't give a s--- whether he's for citizenship at that point. What he's for is enforcement only."
Sharry said passing the security measures without pairing them with reform would require a filibuster-proof Republican majority in both houses of Congress. Even then, Sharry is skeptical Rubio would have the support of pro-reform Republicans. As a result, he said, Rubio's first step would create an impossible barrier to the latter parts of his plan to potentially change the way the US allows immigrants into the country and create a pathway to citizenship for those living in the country illegally.
"He knows that's bulls---," Sharry said of Rubio. "That's what's so maddening to me."
Furthermore, Sharry argued the Gang of Eight bill previously backed by Rubio included substantial security and enforcement measures along with the reform elements. The reason Republicans were ultimately overwhelmingly opposed to that bill, Sharry said, was the path to citizenship.
"I mean, oh my god, the Senate bill had $46 billion in border-security resources," Sharry said. "It required metrics to be met that were extraordinary. It would have an employment-verification system that would be a new labor-market norm and implemented nationwide on a mandatory basis. It had entry/exit systems at the airports that security hawks can only dream of. But they said no to that ... They liked the enforcement. They don't want to say yes to legalization."
Not all immigration-reform advocates are as pessimistic about Rubio. An activist with a prominent immigration-reform group who asked to remain anonymous because of ongoing work with members of both parties told Business Insider it was "important not to discount" the "enormous amount" of work Rubio did on the Gang of Eight bill. Because of this, the activist predicted Rubio would return to a more pro-reform position if he is able to survive the Republican primary.
"I will disagree with a lot of people in the immigrants' rights base," the activist said of Rubio. "I just see this as more as political expediency, and I see him as like more of a future potential ally again than probably other people will."
Still, after Rubio's comments at the Voters First Forum on Monday, the activist was left "shocked."
"Honestly, when I saw that quote - I mean that quote was kind of hard to imagine," the activist said. "It was so bad .... We have to secure the border forever 100%, and we have to finish the fence. We have to do all these things, and then, only after do we think about the legal-immigration system. And then, only after that's totally done do we think about the undocumented. If he's actually being serious about that, that's what - like a 20-year proposition we're looking at here?"
The activist described this as a "fatal flaw" in Rubio's plan.
It would first prevent immigration reform from being enacted under Rubio. And, the activist said, it would have a disastrous effect on the economy, as stepped-up enforcement without reform would cause deportations that would remove "a couple million people from the economy" - including "half of all the farm workers in the country." It would also block businesses from hiring more skilled foreign workers, which many in the tech sector have said is a major concern.
"I think the fatal flaw is twofold," the activist said. "One, like the policy, there is no way, no pathway to success of getting all the components of immigration reform done the way he's laid them out from a coalition policy perspective.
"Secondly, even if I grant some insane possibility that it could get done, the idea that our country is best served by telling 11.5 million people to wait another couple decades before they can get permanent legal status and wait another decade until they can get something? The idea that our agriculture system or Silicon Valley is going to be better off if we say we can't do anything to help entrepreneurs until we can set up this massive interior security deportation mechanism? ... It just doesn't make sense ... It's just bad policy."
While the activist still holds hope Rubio will shift if he emerges from the Republican primary, the activist described his comments at the Voters First Forum as "crazy." The activist also said Rubio's current plan actually made him "worse" than much of the GOP field on immigration reform.
"It's crazy how bad it is. Like really crazy," the activist said. "What he said the other night was really bad compared to almost everyone who spoke."