Where Rubio stands

Among the GOP's announced and expected candidates, Marco Rubio occupies a middle ground. He hopes to appeal to more moderate Republicans, as well as social, fiscal and foreign policy voters. He has credibility with the conservative grass roots after defeating both a Democrat and Charlie Crist, a former moderate GOP governor, in his Senate race, but he offers a less hard-line message than Ted Cruz and Scott Walker.

As a member of the Foreign Relations and Intelligence Committees, Rubio has positioned himself as a hawk, a stark contrast with Rand Paul, who prefers a more restrained approach to military intervention. On Tuesday, Rubio planned to attend a Foreign Relations Committee meeting on legislation to require Congress to weigh in on any nuclear deal reached with Iran.

But his work on immigration illustrates the delicate balance Rubio will have to strike to make it through his party's nominating process. In 2013, he was part of a bipartisan group of senators that drafted a broad immigration bill that included a pathway to citizenship for the 11 million unauthorized immigrants already in the country. He has since distanced himself from the proposal, saying he believes any immigration overhaul must start with securing the nation's southern border and proceed step by step.