Why is Paul Ryan Embracing Immigration Reform?

Paul Ryan has joined Marco Rubio in the battle to pass the Gang of Eight immigration measure.

Ryan offered pub[l]ic encouragement to Rubio and the other “Gang of 8” senators throughout their negotiations to craft a comprehensive reform bill. And unlike other House leaders who encouraged “reform” in theory while dodging the specifics of what a bill would entail, Ryan specifically embraced a path to citizenship for undocumented immigrants right out of the gate.

Is this the re-emergence of Paul Ryan's affiliation with the Jude Wanniski/Jack Kemp school of supply-side economics?

An example of GOP "young guns" unholstering their six-guns together?

Or … more cynically … another contender for the 2016 presidential nomination accepting the preference of the GOP's biggest donors for ultra-slack labor markets?