I draw the line at disrupting the workplace. If you're constantly badgering people about your political beliefs enough to make them uncomfortable, left or right, that's a problem. You can't (and shouldn't) fire someone solely based on their politics, but you absolutely can fire someone for creating a hostile work environment. In most cases, I would talk with the problematic employee with HR and try to get them to fly straight. I'm vehemently pro-choice, but I would still have just as much of a problem with an employee harassing a devout Christian about abortion laws.

What I have zero tolerance for is hate speech. These infractions don't get second chances.