Under what canon?
The definition of heresy is EXTREMELY narrow. The one time I can think of that she broached it, she was resoundingly slapped down by multiple bishops...and hasn't repeated that kind of statement since.
The abortion canon requires the person to actually have an abortion or "procure" an abortion (e.g., perform it or pay for it).
Those are the only two that apply to latæ sententiæ excommunications. The ones I cited above require a trial (i.e., ferendæ sententiæ).
Canon 915 definitely applies. But Canon 915 is not the juridical punishment of excommunication (of course, we will leave aside the fact that the Archbishop of Washington won't even go that far, but that's a different issue altogether).
I’m not a canon lawyer but I think giving public scandal is cause for excommunication. Advocating abortion, in direct opposition to Catholic teaching, is as much a sin as actually having an abortion.