The choice of who or whom is determined by the pronoun’s function within the question.
Who is the subject of the verb was.
When whom functions as an object in a question, it appears out of normal order, before both the subject and the verb. To choose the correct pronoun, you must mentally restructure the question.
Whom is the direct object of the verb did nominate. Restructure: The Democratic Party did nominate whom in 1992?
Exercise: Pronoun case (who and whom) 1
Exercise: Pronoun case (who and whom) 2
Exercise: Pronoun case: review 1
Exercise: Pronoun case: review 2