We've written before about the need to keep political discussions out of the workplace. The best practice is to discourage it - emotions can simply get to volatile, and there's no real need to have that discussion in the workplace, anyway.
And don't let anyone throw the first amendment at you, either: political speech at work is generally not protected by the First Amendment.
Where the political issues get troubling is when employees bring political buttons, signs, bumper stickers, etc. into the workplace.
Make sure to review and/or update your employee handbook to make sure these activities are prohibited (usually through a non-solicitation policy).
And, make sure to read this excellent article on the topic from our friends at Jackson Lewis.