The tea party favorite made it clear over the weekend he has a make-or-break role for the most sweeping immigration changes in decades. It's a high-risk strategy that also puts his presidential ambitions on the line.
Four Republican senators are involved with Democrats in crafting a bipartisan bill that aims to secure the nation's borders, improve legal immigration and offer eventual citizenship to millions now in the United States illegally. But only Rubio has the conservative bona fides plus life-story credibility to help steer the bill through the Senate with strong support from the GOP, and give it a chance in the House, where conservative Republicans hold more sway.
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."
Most popular Nation stories
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 lawbreakers.
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. Now 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.
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 wouldn't comment on Rubio's cautionary remarks.