Try to verbalize the real world facts you are modeling.

"Candidate may be associated with Party". Also true are instances where Candidate is not associated with a party. You can imply this by the lack of relationship between the two instances, (making it a nullable relationship, or zero-or-one to many) or you can introduce a unary fact, "Candidate is independent" with a true/false value.

Physically, a DBA may want to collapse those two facts into one, by making the lack of affiliation to a party one of the values in the party. Carefull though, there are parties out there now called 'The Independent Party', etc.