This was a response on Quora:
Jennifer Ellis, I'm a lawyer and a business woman.
I am going to organize this answer since so many people are reading it.
Generally speaking, here in the US, you have what is called at-will employment. This means you can be fired for any reason, or no reason, barring specific reasons delineated under federal or state law [1] , or if you have a contract or a union.
What are the illegal reasons? At the federal level, they are things along the lines of race, religion, age (over 40), disability and so on.
At the federal level, it is permitted for private employers to fire someone for their political views or their voting choices. It is not legal to fire someone for their political views or their voting choices in a government job. The reason for this is that this would be a violation of the First Amendment. But the First Amendment does not apply to private employers, broadly speaking.
However, some states have passed specific laws relating to employment decisions and punishing someone for whom they vote. California is one such state. In California, you may not fire someone simply because they voted for a specific individual. [2] But, that is not the end of it. The asker suggests that the discussion got rather heated. If the employer could show that the asker was fired because of something he said that was not specific to his vote, or because the employer felt threatened or something along those lines, that would be legal.
I understand that in many countries there are strict laws about firing someone. In the US, this is simply not the case. In the US, you can be very easily fired. And in most states, that includes voting for someone your employer isn’t happy about. But, in most cases, it will not be so simple as, you voted for the wrong person, so you are fired. Why? Well, imagine the public relations nightmare that would come from such a thing.
In the end, my recommendation to the asker is that he speak with an employment law attorney. It is critical that he be honest about what happened so the lawyer can provide an honest assessment about the opportunity for a successful lawsuit.
Footnotes
[1] Questions And Answers
[2] CA Codes (lab:1101-1106)