Twenty-First Amendment Enforcement Act

Amends the Webb-Kenyon Act to authorize a State attorney general (State AG) who has reasonable cause to believe that a person is engaging in any act that would constitute a violation of State law regulating the importation or transportation of any intoxicating liquor to bring a civil action for injunctive relief to: (1) restrain the person from engaging in the violation; and (2) enforce compliance with State law.Grants U.S. district courts jurisdiction over any such action brought by a State AG against any person, except one licensed or otherwise authorized to produce, sell, or store intoxicating liquor in such State. Authorizes such action to be brought in the district in which the recipient of such liquor resides or is found. Limits such action to that seeking injunctive relief (a preliminary injunction, permanent injunction, or both). Directs that such action be tried before the court.Authorizes the court, in such an action and upon a proper showing by the State AG, to issue a preliminary or permanent injunction to restrain a violation. Specifies that a proper showing shall require that a State prove by a preponderance of the evidence that a violation of State law has taken place or is taking place.Prohibits a preliminary injunction from being granted except upon evidence demonstrating the probability of irreparable injury if injunctive relief is not granted, and upon evidence supporting the probability of success on the merits. Sets forth provisions regarding notice to the adverse party, form and scope of order, admissibility of evidence, rules of construction (regarding extension of jurisdiction of Federal courts in connection with State law), and additional remedies.Specifies that nothing herein may be construed to modify or supersede the Internet Tax Freedom Act.Directs the Attorney General to carry out a study to determine the impact of this Act and submit the results within 180 days after enactment of this Act.