Shouldn't everyone be concerned about the Constitution?

Dahlia Lithwick makes a pretty, well, stupid statement in Slate (ironically, while trying to make Christine O'Donnell look stupid):
O’Donnell [in a debate with Chris Coons] explained that “when I go to Washington, D.C., the litmus test by which I cast my vote for every piece of legislation that comes across my desk will be whether or not it is constitutional.” How weird is that, I thought. Isn’t it a court’s job to determine whether or not something is, in fact, constitutional? And isn’t that sort of provided for in, well, the Constitution?
And David Bernstein, writing at Volokh Conspiracy, explains just how dumb of a statement it was:
Short answer: Senators swear an oath to uphold the Constitution. Of course they are obligated to determine whether a bill they are considering is constitutional. Where did Lithwick get the idea that courts, and only courts, should be concerned with the constitutionality of legislation?
The long answer is even better.