re: How are Dems able to stay united, and what can Republicans learn from them?

Dems are single issue oriented, and typically very emotionally involved with that issue. So they are willing to look past any fault in a politician , if he is promising to act on their particular issue. They will be ok with watching the economy crash if abortion, or global warming, or gay rights or whatever their trigger issue is, gets some attention
It makes no sense if you try to use logic to understand it, you never will.