Is Congress Violating the Constitution to Enforce It?

Yesterday the House Agriculture Committee approved a bill that would deny federal economic development grants to projects that involve the use of eminent domain "to obtain property for private commercial development." Unlike the Bond Amendment, which Congress approved in 2005, this bill, known as the Strengthening the Ownership of Private Property Act of 2007, seems to have real teeth. The Bond Amendment, as the American Planning Association happily noted, "contains exemptions [including one for "blight"] that will allow most projects to move forward." The new bill (a version of which was overwhelmingly approved by the House last year) includes just these exceptions:

(A) for use by a public utility;

(B) for a road or other right of way or means, open to the public or common carriers, for transportation;

(C) for an aqueduct, pipeline, or similar use;

(D) for a prison or hospital; or

(E) for any use during and in relation to a national emergency or national disaster declared by the President under other law.

But here's a tricky question for limited-government types who believe in respecting the entire Constitution, not just the Fifth Amendment's requirement that government take property through eminent domain only for "public use," a requirement the Supreme Court has reinterpreted as a an all-purpose license to redistribute land. Does the proposed limit on federal economic development grants (which, it should be noted, are not constitutionally authorized to begin with) improperly trespass on the states' authority to set their own eminent domain policies (within the accommodating parameters set by the Supreme Court)? Is this bill analogous to, say, the highway funding threat that pressured every state to raise its drinking age, and therefore objectionable on similar federalist grounds? Does the motive—in that case, restricting rights; in this case, protecting rights—make all the difference?

I am sympathetic to the view that Congress is stepping in to enforce constitutional rights the Supreme Court has allowed to be trampled. Supporters of the federal restrictions on lawsuits against gun manufacturers made a similar argument vis-à-vis the Second Amendment. In that case, I thought the threat was too remote to justify federal interference with state tort law. With eminent domain, by contrast, the threat is not at all hypothetical, since takings in the name of economic development are happening all the time. Is there a sound constitutional reason why Congress should continue subsidizing them? I think I've pretty much convinced myself that the grant restrictions are OK, but maybe you can talk me out of it.