Would the Pope have the power over members of his church to demand that those members take a specific action? For example, RC's are supposed to support Pro Life policies and politicians. Could the Pope demand all RC's support the pro life politicians or be excommunicated?
Could he? Yes.
Has he already? I can pretty easily defend the proposition that a Catholic who votes for a pro-abort candidate because the candidate is pro-abort, or who votes for a pro-abort candidate when there is a perfectly reasonable pro-life candidate available, is committing a sin. If they understand that and do it anyway, it's a mortal sin.
Persons in mortal sin are not permitted to receive the Eucharist. The difference between such a person and one who is excommunicated is that the excommunicate may need to do more than just repent and go to confession to be back in the good graces of the church. (For example, he may need to make his repentance known to his bishop directly.)
Would it the excommunication you propose be a good idea?
That's a lot more tricky. Because of the different circumstances in different elections in different countries, it would probably have to be done on a case-by-case basis. And *that* would be seen as absolutely reprehensible interference by the Vatican in the internal politics of another country, and you would have the usual hue-and-cry from the usual suspects. Would it be a good idea?
Bingo!