Are we allowed to discipline an employee for spouting off about political topics?

Q.  With the upcoming presidential elections, political conversation among employees has become more frequent and intense. One employee in particular, whose political views are the opposite of most of the others in the office, has taken to vehemently promoting his favored candidate and that candidate’s views. Several employees have complained that this has made them uncomfortable. Can we take disciplinary action against the employee for promoting his political views? We are a private employer.