They already are, automatically. That's what latae sententiae signifies in canon law.
What if the American bishops were to really to do what the pope said?
That's the problem. The episcopate will not enforce the law.
The Vatican should publish an open letter of the excommunication of American pols who support abortion and force U.S. Bishops to deny them sacraments until sincere pennance and a desire to change their ways is desired.