More than anyone else, Rubio, the son of Cuban immigrants, could have the clout to hold off rebellion from conservative talk show hosts and a Republican base whose opposition helped kill immigration changes last time around, in 2007. And perhaps only Rubio could sink the entire effort just by walking away.
If the first-term senator decides against the bill, "that just takes all the oxygen out of the room," said Al Cardenas, chairman of the American Conservative Union. "It may pass the Senate with Democrats' support ... but that's not the kind of support you want out of the Senate if you expect passage out of the House."
With that unique status, Rubio is walking a fine line. He's helping negotiate the politically combustible legislation, which the bipartisan group is expected to unveil next week, while also taking care to maintain the conservative support that makes him so important to the process in the first place.
For Rubio, more so than the other Republicans involved Sens. John McCain and Jeff Flake of Arizona and Lindsey Graham of South Carolina there's danger in a full-throated embrace of comprehensive immigration legislation. For some conservatives, it will always be toxic: It's a priority for Democrats and President Barack Obama that some foes see as granting amnesty to millions of law-breakers.
But Rubio also could see the biggest political payoff. Helping shepherd a comprehensive immigration bill to passage could win support from Hispanic voters that could be critical if he runs for president in 2016. GOP nominee Mitt Romney's dismal showing among Hispanic and Asian voters last November helped seal his loss, and McCain and many other Republicans warn that the GOP risks permanent minority status if it doesn't resolve the immigration issue.
"If he's the guy who helps navigate a reform package over the finish line in a way that brings conservatives along and makes Latinos happy, then his viability as a GOP candidate in 2016 goes way up," said Frank Sharry, executive director of America's Voice, a group that advocates a path to citizenship for illegal immigrants.
All of this helps explain Rubio's caution as the debate moves forward, an approach that was on display this past weekend.
As Graham and Sen. Chuck Schumer, D-N.Y., were appearing on Sunday talk shows to all but declare an immigration deal completed after a hard-won agreement between business and labor on a new low-skilled worker program Rubio was putting out a different message: Not so fast.
"Reports that the bipartisan group of eight senators have agreed on a legislative proposal are premature," Rubio said in a statement that caused some consternation among immigration advocates. Even once there is a bill, he said, it will "only be a starting point," the precursor to what he suggested should be lengthy committee hearings and debate, not to mention full consideration by the American public.
Schumer hurried to dispute the notion of a disagreement between Rubio and himself, calling their difference "semantics" and praising Rubio as an "active and strong participant" in the negotiations.
"He is protecting some of the things that he thinks are very important in the bill," Schumer said on NBC's "Meet the Press." "But I don't think that'll stand in the way in any way of any final agreement."
Obama spokesman Jay Carney said Monday the White House was encouraged by the positive comments over the weekend, but he was far from claiming victory. He said, "The process continues and is not finished." He wouldn't comment on Rubio's cautionary remarks.
Rubio's own comments were only the most recent example of his putting some distance between himself and other immigration overhaul supporters, including Obama and other members of the Gang of Eight.
When a draft of Obama's immigration legislation leaked in February, Rubio declared it "dead on arrival." On Saturday, he released a letter to Senate Judiciary Committee Chairman Patrick Leahy, D-Vt., cautioning against a "rush to legislate" on immigration and asking for full hearings on the bipartisan group's bill once it is released
There's debate about whether Rubio is simply protecting his political flanks, or trying to prepare an exit ramp if he finds one necessary. Aides insist Rubio does want to support a bill and that everything he's doing is aimed at making the legislation politically viable by ensuring support from conservatives.
Schumer and others involved are acutely aware of the importance of Rubio's sign-off. That realization has helped the Florida senator win concessions on border security, legal immigration and other issues during months of closed-door negotiations. "He's gotten his way a good percentage of the time. He's persuaded people to his point of view a good part of the time," Schumer said.
Rubio became part of the bipartisan negotiating group after already having gone public with his own immigration proposals, which emphasize border security first before any pathway to citizenship can begin. And he would make that pathway a challenging one, requiring payment of back taxes and other concessions. Describing his own proposals to a series of conservative media hosts, he won generally positive responses.
Repeatedly, he's made clear that any proposal that does not meet his criteria will not get his support. And he's careful even in how he discusses the proposals under consideration. He shuns the phrase "path to citizenship," which he terms "an inaccurate phrase" because no immigrant can get on a path straight to citizenship they have to get a permanent residence green card first.
Unlike several other Republicans with presidential aspirations, Rubio has avoided getting tripped up on the issue and having to clarify his stance, as happened recently to Sen. Rand Paul, R-Ky., and former Florida Gov. Jeb Bush.
So far, his aides say, Rubio has maintained his support from conservative backers in internal polls and his fundraising remains strong. He made a strong showing at a straw poll at the Conservative Political Action Conference last month, coming in a close second to Paul.
If the final bill is consistent with his principles, says spokesman Alex Conant, "Sen. Rubio and many other conservatives will support it. If the final bill doesn't meet those principles, Sen. Rubio won't be able to support it and conservatives will appreciate his principled stand."