Generally, having a schema representing an entity to have any one of a given set of attributes is not considered good ER-design. (because that's not how intuitive object-oriented systems work, in real world).

An alternative solution is to have an attribute (boolean) "Independent" associated with the Candidate, and an attribute "Party" associated too. The value in attribute "Party" is used only if the attribute "Independent" indicates the candidate belongs to a party.