County-by-county psephological maps show that your base is now confined to two major coastal enclaves and a handful of university towns. Only 4 of 50 states have both a Democratic-controlled legislature and a Democratic governor. In 2018 that regionalization is going to get worse, not better; you will be defending 25 seats in areas where Trump took the popular vote, while the Republicans have to defend only 8 where Clinton won.
Your party leadership is geriatric, decades older than the average for their Republican counterparts. Years of steady losses at state level, masked by the personal popularity of Barack Obama, have left you without a bench to speak of – little young talent and basically no seasoned Presidential timber under retirement age. The fact that Joseph Biden, who will be 78 for the next Election Day, is being seriously mooted as the next Democratic candidate, speaks volumes – none of them good.
Your ideological lock on the elite media and show business has flipped from a powerful asset to a liability. Trump campaigned against that lock and won; his tactics can be and will be replicated. Worse, a self-created media bubble insulated you from grasping the actual concerns of the American public so completely that you didn’t realize the shit you were in until election night.
Your donor advantage didn’t help either. Clinton outspent Trump 2:1 and still lost.