Congress has the power to abolish all copyright laws and to abrogate any treaties that compel it to recognize copyright laws.
But, under the 5th Amendment to the United States Constitution, private property may not be taken by the government without just compensation (subject to exceptions not applicable in this fact pattern). So, the U.S. government would have to buy out the fair market value of all copyrights currently in force in order for it to do so, and it would have to do so in a manner that affords copyright owners reasonable due process in connection with the making of that determination.
In the alternative, the U.S. could, for example, eliminate copyright protections prospectively to newly created works, while allowing existing copyrights to remain in force.
Also, while the U.S. would have to at least allow copyright holders to obtain compensatory damages for breaches of copyright, it could probably also immediately extinguish all rights under copyright to statutory damages, attorney fees, criminal sanctions for copyright infringement, injunctive relief, and punitive damages for copyright infringement. It could also probably impose reasonable procedural requirements requiring claims of grandfathered copyrights to be documented or registered within a reasonable period of time.