Well, let's consider what you are saying. Suppose the Congress were to pass a law making it a felony to own or possess a firearm (even in the home) and a provision requiring federal officials to begin confiscating firearms. Let's suppose that the President then vetoes the statute because he believes that the statute is unconstitutional. Let's suppose that the Congress then overrides the President's veto by passing it again with the support of 90% of the members of Congress.
Would the President be acting appropriately if he refuses to confiscate weapons because he believes the statute to be unconstitutional?
If it would be appropriate for the President to refuse to enforce the statute by declaring it unconstitutional, why would it be inappropriate for a court to refuse to apply the same statute by declaring it unconstitutional?
In other words, why should it be assumed that the Congress should have the last word in determining constitutionality?
Since the President has sworn to preserve and protect the Constitution, isn't he required by his oath to refuse to enforce that statute? And, aren't judges (who take a similar oath) required to refuse to treat that statute as a valid law?
These are some of the problems that result from a branch of government exceeding its constitutional authority. I don't claim to know what the proper answers are, but I know that those answers aren't obvious or simple. If the drafters of the Constitution had included language that required the executive and the judiciary to enforce and apply any statute passed by the Congress (whether constitutional or unconstitutional), I am not at all sure that we would have a system better than the system that we have right now.
Would the President be acting appropriately if he refuses to confiscate weapons because he believes the statute to be unconstitutional?
If the President has a reasoned opinion from his Attorney General that the statute is unconstitutional, then, yes, he should refuse to enforce the law.
why should it be assumed that the Congress should have the last word in determining constitutionality?
I do not assert that Congress should have the last word. However, if one of the three branches must have the last word, then better Congress or the President than the Court because:
a) Since governments derive their just powers from the consent of the governed, Congress and the President are more legitimate repositories of power because they are elected, as opposed to the judges who are not elected; and
b) The people have a chance to change Congress and the President frequently and regularly, whereas judges are appointed for life.
Thus the Court is the least suitable of the three branches to have the last word on what the law is.
Rather than give any of the three branches the last word, however, it would be better to give the people the last word within the framework of the republican government established by the Constitution. See the example below.
Since the President has sworn to preserve and protect the Constitution, isn't he required by his oath to refuse to enforce that statute?
As per above, yes.
And, aren't judges (who take a similar oath) required to refuse to treat that statute as a valid law?
Yes. The judges can refuse to give the law validity in deciding a case or controversy. They cannot, however, declare the law invalid for other parties or future cases.
To work with your example: Suppose the Congress were to pass a law making it a felony to own or possess a firearm (even in the home) and a provision requiring federal officials to begin confiscating firearms. Contrary to your example, however, let's suppose that the President signs and enforces the law.
Suppose now Hapless Harry is arrested and convicted for possession of a firearm. He appeals all the way to the Supreme Court, which reverses his conviction. Harry is now free. However, the law is still on the books and the President can still enforce it.
Suppose Luckless Larry is arrested for possession of a firearm. The trial court may refuse to convict, following the Supreme Court, or it might convict, in which case Larry can appeal and ultimately have his conviction reversed. But, the law is still on the books and the President can still enforce it.
Suppose now Conceal Carrie is arrested for possession of a firearm. The trial court may refuse to convict, following the Supreme Court, or it might convict, in which case Carrie can appeal and ultimately have her conviction reversed.
The question will soon become: who is first going to get tired of this fol-de-rol? The legislature could repeal the law; the executive could cease to enforce it (which raises the question: may the President refuse to enforce a law which he believes, supported by a reasoned opinion from the Attorney General, is constitutional, or does the AG have to revise his opinion?); or the judiciary could change its opinion of the constitutionality of the law (a more likely occurrence in the case of a 5 to 4 decision).
All of these questions will devolve into a question of the support of the people. If a large majority of the people really want this law, then the executive, and the legislature which passed the law, will feel that support and persevere. On the other hand, if a large majority of the people don't really want this law, then the executive and the legislature will feel it at the ballot box. In either case, it becomes a political question, rather than having the Supreme Court act as super-legislature telling the people how they may govern themselves.