The key to 2016 is probably the upper MidWest and states like Colorado out west. If we accept that most likely the dem nominee isn’t going to get the high rate of minorities like Obama did then the key would be white middle class voters
The key of course being the Democrats can figure out a way to attract enough white voters - particularly males to be competitive.
They have maxed out the minority/female vote as it is and its unlikely to get higher. If it declines, they won’t be where they need to be to win.