To demonize someone is to characterize them as evil or wicked, whether or not they actually are. It's distressingly common in politics for one party to demonize members of the opposing party.
This verb literally means "to make into a demon.” Demon means "evil spirit," so when you demonize someone, you portray them that way. This tactic shows up in propaganda and politics all the time. If you’re running for class president, don’t demonize your opponent just to get votes — instead, engage in cordial debate about the issues.