999 and the Constitution

About the same as Obamacare and the Constitution: as far as I can tell, there is no enumerated Federal power to impose a national sales tax, or a value added tax. And the 10th Amendment says, in toto: The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people. So then, where would the authority of the Fed to impose a sales tax come from? The Commerce Clause? But that's what ObamaCare tries to do, and that's precisely what will be adjudicated by the Supreme Court next year. Do we want to undercut that challenge? On what grounds?