What the Fallacy?!
Guilt by Association
The Fallacy
Attempting to discredit an idea or argument by associating it with an undesirable person or group.
Why it's wrong
This fallacy assumes that because a person or group is viewed negatively, any idea they hold must also be incorrect. However, bad people can hold true beliefs (like 2+2=4), and good people can be wrong. The validity of an argument is independent of who supports it.
Example
The candidate argued for better highway infrastructure. His opponent dismissed the plan entirely, pointing out that a notorious dictator from the 1940s also built highways.