Behind the scenes, Rubio has bolstered his political action committee, Reclaim America PAC, to help him raise money, elect fellow Republicans and essentially create a campaign-in-waiting. Terry Sullivan, a trusted aide with deep ties to the early primary state of South Carolina, has moved over to work full time at the organization, and Rubio hired Dorinda Moss, a leading GOP fundraiser, to be its finance director.
Rubio, who took hard-line positions on immigration policy such as branding Sen. John McCain's proposal in 2010 a form of amnesty, has shifted gears on the issue. He spent months meeting with different groups in the debate before releasing a set of principles with fellow senators that pairs increased border security with a possible path to citizenship for an estimated 11 million illegal immigrants.
Mindful that "amnesty" remains a dirty word in the GOP and that many still flinch at immigration changes approved under President Ronald Reagan, Rubio courted conservative talk-radio hosts and news outlets to head off criticism. Early on, he won positive reviews from powerful conservative media personalities.
Fox News' Bill O'Reilly and Sean Hannity offered praise for his approach. Conservative commentator Rush Limbaugh called Rubio's ideas "admirable and noteworthy," a tag line that could turn into a handy defense in a Republican primary.
Rubio has had detractors, too.
Conservative commentator Ann Coulter said Rubio's plan would allow illegal immigrants to live and work in the U.S., essentially jumping ahead of those who have waited in their own countries to immigrate to the U.S. legally. Louisiana Sen. David Vitter, a Republican, called Rubio "naive."
Indeed, the focus on immigration could be a boon or a bust for Rubio. If he rallies Republicans behind a plan that offers some resolution, he could be connected to a big legislative victory. But if the plan draws the wrath of Republican activists, it could hinder his ability to seek the big prize in 2016.
"I really believe that if I do the best job I can in the Senate," Rubio told BuzzFeed, "then in a couple of years I'll be in a position to make a decision about whether I want to run for re-election, leave politics and give someone else a shot or run for some other position."
Associated Press writer Christine Armario contributed to this report from Miami.
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