So who the candidates are is more important than the size of the base when it comes to presidential elections. Also whom the voters are mad at or happy with. In 2008 the voters were just plain tired of Republican rule, Obama won 53% to 46% over McCain, a 7 point margin. Yet party affiliation was 36-25% Per Pew Research a margin of 11 points in favor of the Democrats. In 2012 the numbers were 32-24% Democratic advantage with Obama winning reelection by a 51-47 margin.
Section 9: Trends in Party Affiliation | Pew Research Center for the People and the Press
Today I do not have figures from Pew, but I do Gallup and as of 9 November 2014 Gallup places party affiliation at 28% each for the Republicans and Democrats. My point is your demographics are included in these numbers, they take the entire electorate and break them down by party. I think that is a much simpler way.
As for Hillary or Warren or whomever, any Democrat will be very hard to beat. This is because of the Electoral College. Obama beat Romney by 4 points in the popular vote, but by 62-38 margin in the Electoral College. Unless Obama fatigue sets in big time like Bush fatigue did between 2006-08, regardless of whom the Democrats run they start out with a 247-191 trustworthy states advantage in the Electoral College. But that can change based on whom the candidates are and on whether Obama fatigues lasts another two years and how the economy is doing.