Is it proper for a legislator to consider the constitutionality of a bill when voting on it?

Dahlia Lithwick thinks not, apparently:

I have been fascinated by Christine O'Donnell's constitutional worldview since her debate with her opponent Chris Coons last week. O'Donnell 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?

More here.