A nice map from dreaminonempty at Open Left illustrates the vote share won among white men in the 2008 presidential election:
The take home message: expanding voting rights – a progressive position – resulted in the ability to elect more liberal politicians.
I would say that another message is that progressive politics is badly disadvantaged by a situation in which the overwhelming majorities of political leaders and prominent media figures are white men. There are plenty of white men with progressive views, but in general the majority of white men are not progressive and the majority of progressives are not white men. Drawing from the relatively small pool of white male progressives means drawing from a shallow talent pool.