This is not entirely true though. Each party does get a bunch of "blind" votes, but they represent the minority, not the majority of what elects a president. That's why think tanks on the left and even Obama himself said that they lost the 2016 election because they ignored the moderates and the middle class.
They know this too, which is why Joe's campaign is scrambling to constantly be pictured as a "moderate" president even though his platform is literally as far left as you can go right now and why they openly continue to support far left movements.