The biggest challenge Democrats have had is that they want their policies to be good for everybody, even the people that hate them. This means democrats aren’t as excited about the policies and Republicans just hate everything they do no matter what.
The ACA was the perfect example of this. Dems could have done whatever they wanted, but chose a very conservative healthcare model. Dems were indifferent and Republicans just latched onto this as a lightning rod for their hatred.
Republicans on the other hand will give their base 100% of what they want with no compromise. If you didn’t vote for republicans, they don’t care what you want or how you feel. This gives their base massive enthusiasm.