This question presumes the following circumstances:
A party files for declaratory relief in federal court requesting a state statute be declared unconstitutional (facially or as-applied) but chooses not to seek injunctive relief aimed at preventing its enforcement. Judgment is entered for the plaintiff and the state doesn't appeal. Later the state prosecutes the plaintiff in violation of the decree (which it can do as the decree itself is non-binding). I am curious as to what recourse the plaintiff would have in this situation. Some states have followed the federal government in extending collateral estoppel to criminal actions and certainly in that circumstance the plaintiff could challenge the constitutionality of that statute in the state criminal proceeding and then estopp the state from defending against that challenge on grounds this issue had already been litigated. In the event that is not an option I am curious as to how Younger abstention would apply. The anti-injunction act would not apply as 42 USC 1983 explicitly permits federal courts to enjoin a state judicial officer when "a declaratory decree has been violated". The Court appeared to be somewhat split on this issue in Steffel v Thomson and explicitly reserved judgment on this question to a later date and I am unaware of any subsequent case law on that issue. Under 28 USC 2202 a federal court can issue all "necessary and proper relief" to enforce a declaratory judgment against an adverse party "after reasonable notice and hearing". However, my remedies text states that one has to file a separate suit to obtain an injunction under that statute. It provides no reference as to why. In fact, the U.S.C. notes for 2202 suggest that the intention was that the court would issue an order to show cause as to why the "further relief" requested should not be granted. Why couldn't the plaintiff just file for an injunction (under 2202) ordering the termination of the criminal proceedings under the original title and case number as a request for post-judgment relief? In that instance Younger abstention wouldn't apply as the suit would have predated the prosecution.
EDIT-- I'm going to clarify my question in response to a comment. I am not concerned with collateral estoppel, I just noted in passing that would be one possible recourse (federally it is automatic). The question presumes this is not an option. Younger abstention states that a party cannot pursue an injunction (in federal court) prohibiting the enforcement of an unconstitutional state criminal law once a criminal action has commenced against the would-be plaintiff for its violation (Younger v. Harris). In fact, a district court is required to abstain even if the civil case challenging the statute preceded the criminal action so long as the case is in its infancy.This would be a potential problem in the following sequence:
1.) Party A seeks declarative relief (and elects not to also pursue an injunction enjoining the government from enforcing it) and wins. The Court issues a decree stating the criminal statute is unconstitutional and that Party A has the right to engage in the conduct at issue in the case. There are good reasons that a person would choose to do this.
2.) Party A engages in said conduct, the declaration is non-binding and so the government elects to prosecute.
3.) The law says the Court may choose to issue an injunction to enforce a declaratory judgement (say by enjoining the government to terminate the criminal proceedings against Party A). However, if this requires filing a new pleading (and therefore technically a new case) by Party A, this case requesting enforcement of the declaration would follow the commencement of a criminal proceeding, and it seems reasonable that the Court be required to dismiss the action under Younger. However, I don't see any reason why this request should not be pursuable in the original action. The law only requires "reasonable notice and hearing" be given to the party that lost the declaration. It is not uncommon for persons to seek ancillary orders as post judgement relief in a case. I don't see why a NEW case needs to be filed to enforce the declaration. If the injunction can be pursued in the original case than Younger abstention is most definitely not triggered (the original case preceded the filing of charges),and the judge could order the prosecution terminate the criminal proceedings. I'm having a difficult time finding a definitive answer on this as almost always the injunction is pursued simultaneously with the declaration.