Last time I checked only the supreme court has the authority to declare something unconstitutional.

That is not entirely correct. They have the final say in determining what that is, but anyone can declare someone unconstitutional based on prior Supreme Court decisions.

The whole US federal legal system is based on that since you first need to get a judgment in trial court, then appeal and only then you can get to the Supreme Court (in 99.99% cases as there are narrow exceptions).