Declaratory judgments still require two actual parties, and an actual (or at least substantially likely) dispute between the parties. The Constitution only grants Federal courts jurisdiction to hear "case[s] or controvers[ies]" - without an actual dispute, it would be an advisory opinion, which courts (Federal courts, at least) are unable to render.
At the moment as I recall Cruz has a nuisance lawsuit or two against him. So there is his other party. Or he could wait until he wins the nomination and then the big Dem legal guns come after him.