Excellent question, and one that invites consideration of the entire development of American law and the western legal tradition on which it is based.
The answer is not a straightforward "yes" or "no."
Typically, juries decide fact disputes based on instructions about the applicable law given to them by judges.
In the two examples you mentioned, Prop 8 & Roe v. Wade, judges were called on to make rulings on motions for orders preventing enforcement of state laws. These motions for injunctions ask the court to decide that the relevant law is incapable of application in any set of facts without a violation of the Constitution. The decision to grant or deny such a motion reflects one of two main aspects of judicial action. Judicial action general categorizes as equity or law. Equity powers of our courts arose in England when religious tribunals were called on to render orders to repair injustices. Typically, equity involves ordering a party to do a thing, or prohibiting a party to do a thing (injunctions), or ordering government officials to undertake action required by law (mandamus) or refrain from action prohibited by law (prohibition). Law powers are the ones typically associated with civil cases involving determining whether a party has been damaged and ordering the payment of money damages therefor.
Although I have never seen it in 25 years of federal civil rights and constitutional law practice, judges can empanel an advisory jury in in the federal courts when ruling on motions for injunctions.
One final note, even though judges "instruct" juror about the law, the principle of jury nullification recognizes that juries have an organic power to decide that a law is wrong and find a defendant not guilty of violating it.
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My colleague gives a more complete answer, but briefly: juries decide facts and credability; judges determain the application and determination of law.
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Whethere or not a statute or law is constitutional in a criminal case is a matter for the judge to decide. That is a question of law.
In my jurisdiction we can challenge the constitutionality of a law or rule in general or as applied to the client's case. This is a matter that is litigated with the judge prior to trial in a motions session. Depending on the judge's decision, then the matter becomes an issue on appeal.
What the jury decides is whether the facts support a result based on the law as told to them by the judge in the jury instructions.
There is a concept called jury nullification where the jury might find someone not guilty despite the facts and the law.
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