For decades young adults have tended to lean slightly Democratic but now the preference is overwhelming. Democrats have an unbelievable 18 point advantage among the current youngest generation. This is unlike anything in the past 20 years. From Gallup:

Party ID by Generation -- 1993, 2003, 2013

This is the dynamic that is going to dominate politics in the coming years. On many issues Millennials  are dramatically to the left of the rest of the American public. The generation is often even more progressive than self-identified “Liberals.”

Republicans’ current opposition to issues like marriage equality is so abhorrent to many young people the party isn’t even seen as a possible election choice. The Republican party is either going to need to change or get buried in a generational tide wave.