I think the angle Dems in congress will take is to make the circumstances that made Kyle able to legally possess the AR in Wisconsin a federal crime. In place, the Dems hope there will be new federal gun laws to prohibit states from reciprocating gun laws, make it illegal for an Illinois resident to possess and use guns in another state and bans on “assault weapons.”
Even if the Dems try it and actually pass a new law, it would not have been a federal crime at the time of the incident.