6.75 million registered democrats
2.9 million registered republicans
3 Million independents
Simple math
Given those numbers even if every I and R voted for the Republican the Democrat has 55% of the vote
The math is only part of it. The party which can energize its base the most will prevail, even if in the minority. And Democrats have mastered how to energize their base - mostly with lies.
Dems are MUCH more likely to vote than Republicans, in my opinion. That's why in even conservative areas, Dems dominate big city politics. It's low hanging fruit for them.
How many of the D’s either work for the government or are subsidized by the government of the 55%?