To understand his options, it helps to think about the potential outcomes:

1) Supporting a Bill That Passes. Rubio is a real-deal political talent, but as a city councilor, state legislator and now U.S. senator, he’s got a thin legislative record. Now he’s probably the most important player working on a big bipartisan deal. Brokering a reform bill that helped fix the nation’s immigration problems and the GOP’s Latino problems would be a huge achievement for a freshman. It could disappoint grass-roots conservatives who tend to dominate Republican primaries, but so far Rubio has gotten sympathetic hearings from Rush Limbaugh, Sean Hannity and other talkers who helped rally the base against reform in the past. And GOP elites are desperate to get the immigration issue off the table; they’ll be deeply grateful to Rubio if he can help make that happen.

2) Opposing a Bill That Fails. The media love bipartisan gangs, but even if the Senate negotiators reach a consensus, it’s not clear that immigration reform, especially a path to citizenship, can pass the Republican-controlled House. So Rubio might be tempted to get off the legislative train before it derails, and perhaps even get credit for killing the bill. He could always argue that Obama was too squishy on enforcement, or too union-friendly on guest workers, or whatever; he’d be a hero to the base for standing up to the sharia socialist, and he could still get some mainstream-media credit for supporting reform as long as he did. Advocates warn that if Rubio scuttles the bill, he’ll become as unpopular with non-Cuban Hispanics as Mitt Romney was, but that doesn’t seem plausible. If 2016 rolls around without reform, Republican elites will still be desperate to solve their problems with Hispanic voters; wouldn’t a bilingual Hispanic-American who took 55% of the Hispanic vote in a swing state be an attractive nominee?

3) Supporting a Bill That Fails. Rubio may be the key senator when it comes to reform, but he’d rather not help pass a bill that stalls in the House. He’d get no achievement to put on his resume, he’d look impotent, and he’d be linked to an Obama initiative rejected by the Republican base. This could conceivably make him an even more attractive candidate for 2016, when the problem would still be lingering and Republican elites would be even more desperate for a pro-reform, Hispanic-friendly nominee. But it would be embarrassing. Rubio doesn’t want the limb sawed off while he’s still on it.