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.