Rand Paul: If the first three months of the 2013 Congress have proved anything, it's that the Kentucky Republican with one eye squarely on the 2016 presidential race has a real ability to influence issues and debate in Washington. (See drones.) In a speech to the U.S. Hispanic Chamber of Commerce last month, Paul seemed open to a comprehensive overhaul although pointedly did not use the phrase "path to citizenship," which is sine qua non for Democrats when it comes to the broader bill. (Paul is apparently for a path to normalization, not a path to citizenship.) If he signs on to whatever the final bill looks like, Paul will provide significant cover for conservative Senators wary about voting in favor of something that a significant chunk of their GOP constituents oppose.
Mark Pryor: The Arkansas Democrat is up for reelection in a very red state — President Obama lost it by 23 points in 2012 — and will be very mindful of concerns that this bill rewards those who entered the country illegally. (If you haven't bought stock in the word "amnesty," now would be a good time to do so, because you are going to hear it a lot over these next weeks and months.)
Rubio: Yes, the senator from Florida gets lots and lots of attention on the immigration bill. (He was on seven — yes, seven! — talk shows on Sunday.) But, in this case, the attention is warranted. Rubio not only is Cuban-American but also represents a state where immigration is a top-of-the-mind issue. And then there is the fact that he is widely regarded as the front-runner for the 2016 Republican presidential nomination. It's no exaggeration to say that if Rubio walks away from the final product on immigration, the bill might not make it.