From Prof. Eugene Volokh:
There are a host of special rules that simultaneously constrain and empower equitable remedies. These rules include the irreparable injury rule, equitable ripeness, equitable defenses (e.g., unclean hands and laches), opportunities to revisit and reopen the remedy (e.g., modification and dissolution of an injunction), and enforcement mechanisms such as contempt. These special rules apply to — and ordinarily only apply to — equitable remedies. They do not apply to the declaratory judgment.
Here are four thoughts:
(a) The idea of the declaratory judgment is very old even if the name is new. Two people have a dispute over rights and they ask the court to resolve it.
(b) A declaratory judgment is not a remedy. Getting one does nothing directly to solve the plaintiff’s problem. A remedy is a command, even if just an implicit command in the sense of declaring someone owes damages.
(c) A bit separate: ordinarily the plaintiff asks for an injunction as well as a declaratory judgment. The DC Circuit Court of Appeals ruled en banc in the Cohen tax case (2011) that despite the plain language of the Declaratory Judgement Act, its scope is the same as the Anti-Injunction Act’s, so declaratory judgments which do not restrain the collection of taxes are okay. The court noted that this was perhaps of little practical importance, since it is the injunction that is the main remedy anyway.
(d) Also separate: I found this in a footnote: “Abbott Labs. v. Gardner, 387 U.S. 136, 155 (1967) (the declaratory judgment and injunctive remedies are equitable in nature” But the Supreme Court was wrong when it said this.