Any attempt to declare the congressional seat of Rep. Gabrielle Giffords (D-AZ) to be vacant based on state law would probably violate the U.S. Constitution, the Tucson Sentinel reports.
Though Arizona statutes say public offices may be declared vacant if their holders fail to discharge their duties for 90 days, it pertains only to state and local officials.
Washington Post: "The U.S. Constitution provides the qualifications for service in Congress and makes the House the sole judge of those qualifications. Courts have consistently held that states cannot add qualifications to those in the Constitution and have rejected efforts to remove members of Congress, even through term limits and recalls."