I'm not up on the current movement to petition the Church to make a pro-abort stand (as taken by a public politician) grounds for excommunication so I can't really speak to the arguments they're using.
I do know that such a movement would unfair preclude the "religious" -- such as the radical nuns behind "Network" -- from being excommunicated as they are careful always to avoid the subject of abortion as they seek to influence the political views of ignorant faithful by selectively quoting from legitimate Catholic teaching.
My guess is that such partaking of the sacraments -- if truly in error due to the malformed conscience or ignorance of the Catholic -- is a case of "invincible ignorance". Where the Catholic is purposefully disobeying his conscience and the teaching of the Church both with respect to abortion AND the partaking of sacraments while under a cloud of grave or mortal sin ... I think the act could be described as "unlawful".
This is how a Mass would be treated if the priest took liberties with the liturgy clearly outside the precepts of the sacrifice. The Mass is still valid but is has been celebrated "unlawfully".
I suppose by this reasoning, one leaves room for the operation of the sacrament (affording the necessary graces by which a Catholic in error might right himself) but under a cloud of illicitness that attaches only to the Catholic himself, not the sacrament.
I'll bet one of the other more well-catechized Catholics might know better than I. So, I'm pinging Eastsider, Romulus and Campion for starters.