Cato's Will Wilkinson argues that younger, more socially liberal voters are the prime target for a future "liberaltarian" left-liberal fusion:

[O]lder people’s politics look a lot different from younger people’s not so much because they are older, but because they are part of a different generational cohort. Different generations grew up under different economic and cultural conditions, and this affects their values throughout life. It’s exceedingly unlikely that today’s 20-somethings will ever be as socially conservative as today’s 60-somethings. There are life-cycle effects, but the age-related changes tend to tend to stay within the gravitational field of relatively stable points fixed in adolescence and young adulthood. ...
One thing it suggests that the neo-Fusionist elements of the Tea Party movement are attractive primarily to older people. And I suspect that the more strongly certain libertarian ideas and tendencies are associated with the cultural politics of Baby Boomer conservative Republicans, the more strongly young people with libertarian inclinations will tend to identify with the Democratic Party and take on cultural assumptions and characteristics common to liberals.

Wilkinson is probably right about cohort effects -- people who are 20 years old today have more liberal views on things like interracial marriage and gay rights than people who are 70 years old today, and in fifty years, their views probably won't change much. Wilkinson, though, errs in his assumption that this will make those voters long-term members of a socially liberal electoral coalition. His error is  ignoring the possibility that the issue landscape will move leftward.

Generally speaking, social policy has grown less restrictive throughout American history. People are are currently 70 years old are far more liberal on, say, the question of women's suffrage than were people fifty years their senior. But women's suffrage is settled fact, and no longer exerts any electoral impact. Several decades from now, we may well be looking at an even more liberal or left-wing social issue landscape. Today's young voters are much less freaked out about an African-American president than are today's old voters. But what about fifty years from now when the Democrats have nominated a transgendered Presidential candidate?