You can vote for a party and still disagree with some of their policies.
https://www.nytimes.com/2016/12/03/us/maine-ranked-choice-vo...
The reason I'm invested in the U.S. political system is that it produces something resembling international jurisprudence ("if the U.S. is doing it, we should too!"), because it influences our policies through trade agreements and the U.N., and because many U.S. companies are very influential internationally.
That, and Trump's climate policies affect every conscious being on Earth.