Synonymous is an adjective from the word 'synonym'. It means that the two things being compared have the same or nearly the same meaning. For example, 'big' and 'large' are synonymous- they are synonyms. In this sentence the writer is saying that Nixon and political scandal are so closely related in the minds of the people (due to the scandals around his presidency) that they now mean nearly the same thing.

A synonym is a word that has the same meaning. So nixon's name is the same as saying political scandal.