I don’t know about all states and if its federal law or not (and I could look it up but today I’m lazy), but you can be fired in this country based on your political opinions and affiliations. So expressing your political opinions online somewhere like Facebook could harm your career track, especially using your real name on Facebook; using a business profile and expressing opinions is bad marketing.
On the contrary, in the days of Glassdoor, and Indeed and employee reviews it’s probably not a shrewd move as an employer to fire someone because they like Trump or Sanders or whoever, You may subsequently find that the only people you can hire in the future are people that nobody else wants to hire. Glassdoor and Indeed employee reviews are no longer just sour grapes and they never really were, because if you understand the business side and can read around the obvious business model patterns, it’s very easy to deduce from Glassdoor employee reviews how well a company is being lead. If you ask Warren Buffet, leadership is a very big part of how he decides whether to buy stock in a company or not – it’s all about, the leadership. When using websites like Glassdoor and Indeed, sometimes it’s not about the existence of information or not, but it’s a lot about your understanding of that information and perhaps how you use it for yourself.