(I am not Catholic, so I do not fully understand excommunication)
However, it appears that the Pope has made it clear, support for abortion is grievous enough to deny communion, it would also seem to follow excommunication.
That being said: I am evangelical...and it doesn't matter whether you are RCC, Evangelical or Mainline Protestant...we ALL have those people within our midst and within our pastoral heirarchy...and I despise them all. There are just as many people within Protestant/Evangelical circles who hold the "pro-LIFE" stance but their knees get weak around power...and they tolerate Jezabel within their midst and wouldn't even dream of following 1 Cor 5 and separating that person from them.
I once had lunch with Nick Lampson. He knew I was a preacher and was being real chatty. I could tell he was looking for a dim "in-road" to the "faith" vote in the district he was carpet-bagging in (his 3rd or 4th now). I looked at him in the eye and told him I wouldn't let him...or ANY politician...not even George Bush (this was in 2004) into my pulpit unless he was there to preach Christ and Him crucified...and that NO politician, R or D, would ever have my support if they held to the pro-choice stance, whether by voice or vote (including electing a pro-choice speaker)...and I would publically condemn anyone who HAS that opinion. It shows that one had no moral compass...and the last thing we need in DC are people whose moral compass' are broke.
Simply, excommunication means being obliged not to receive Communion.
ANYBODY who commits a "mortal sin" is obliged not to receive communion (except in an emergency) unless he goes to confession and is absolved.
Some deeds are singled out. Having or procuring an abortion,for example, makes you excommunicated.
Here's one place where canon law is different from what we're familiar with. In such a case you are excommunicated latae sententiae -- automatically. You know what you did, you know the law (theoretically) so there's no need for a formal inquiry and declaration.
But you can also be officially declared excommunicated. That would be different. So if somebody says, "anyone who advocates or requires abortion is excommunicate," or even if a bishop, say the Bishop of San Francisco, asks a politician into his office and says,"You know, you really should not receive communion," it's not official. It is on the individual, and the divine penalty is on him too.
Also, there are misdeeds which automatically excommunicate and which require the Holy See (the pope or his agents) to lift the excommunication. A priest who breaks the seal of the confessional can only be readmitted to communion by the Holy See.
The way the canons are written is unusual. The relevant canon, 912, says
Can. 912 Any baptised person who is not forbidden by law may and must be admitted to holy communion.SO that's like the default position. A minister of Holy Communion must assume, unless he KNOWS otherwise, that the person in front of him "may and must" receive.
SO, unless it's official, the minister has to assume that, however unlikely it may be, the person in front of him went to confession and was readmitted to the sacrament before he stood there and said, "Amen."
I don't know the official procedures for declaring someone excommunicate AND declaring the lifting of that status as "reserved." But it would take some sort of formal declaration not only that the person was excommunicated but that the excommunication could not be lifted by some random confessor. THEN the minister could deny the sacrament.
One way to look at this is that the canons bend over backwards to make provision for God's acting mercifully in the life of the sinner, and also that they are based on the idea that God knows what hes doing even when we don't.
It is not possible for the Vatican of the bishops to publicly excomminicate all the silly apostate Catholics who endorse anti-life policies that are completely contrary to the Catholic doctrines and indeed, biblical thruths since the time of the New Testament.
However, it would be possible for the entire Bishops conferance to anounce that any such dissenting Catholics should not approach the altar to receive holy Communion.
It is the belief of the RC church that to receive the Holy Eucharist knowingly with a sinful heart [i.e. with grave sinupon your sould] is to call down upon your soul the risk of eternal damnation. A publication of the sins involved which are pro-abortion, adultery, pro-euthanasia, acceptance of homosexual behavior or Gay marraige, etc. needs to be done often and repeatedly until all Catholics understand the gravity of their actions upon the destiny of their immortal souls. Such persons excommunicate themselves and it behooves the leaders, the bishops, to proclaim it publicly.
It mighr be a good idea as well to give examples of some famous folks like Nancy Pelosi, Joe Biden, Andrew Cuomo, as examples of persons whose actions have excommunicated themselves.
God and his Son, Jesus Christ will not be mocked!
Banning the person from receiving communion is because they have excommunicated themselves.