I am less worried about the demographics than you are, admittedly.
Nominate a Republican candidate who can get the number of votes McCain got, for starters. Romney was leaving votes on the table that were obvious for him to get. Those people were not voting for Obama. Obama’s votes were way down.
The battle is going to be over all those people who were in the decreased turnout this year and it’s not at all clear that they were Democrats.
Just trying to give another point of view. There was nothing extreme or “landslide” about this. Obama eked out a small victory, and he was the incumbent so he starts with that advantage, which the next Democrat candidate won’t have.
And as you mention, the left can fragment — it commonly fragments. This unity behind Obama is the exception, not the rule. People have a particular emotional attachment to him they’re not going to have to, say, Hillary.
Regarding demographics: You should be very worried.