Why is this phrase proper in using the correct form or be and have in the present tense? "I am interested in working on the presidential campaign if the opportunity still exists."
"am" = present tense
"exists" = present tense
If you use other tenses in either place, the sentence won't have correct (or understandable) verb tense sequence.
http://grammar.ccc.commnet.edu/grammar/verbs.htm#sequence