If Kerry’s church was not in communion with Rome, then of course the RC bishop would have no authority over him or his church. But that’s not the case, since Kerry’s church is indeed a regular Roman Catholic church.
Sean O’Malley is the archbishop of Kerry’s diocese, and as such he has the authority to prohibit Kerry from receiving communion. Indeed, he has stated that a Catholic politician who publicly supports abortion rights is in grave sin, and should therefore refrain from communion (this follows from the Catholic principle that communicants should not be in a state of sin; if you’ve commited a mortal sin and haven’t received absolution, you’re commiting an even graver offense by going on to receive the Eucharist). But it’s up to the individual politician to recognize this–Archbishop O’Malley has also said that “the church presumes each person is receiving in good faith. It is not our policy to deny communion. It is up to the individual” (cite).
Archbishop O’Malley has clearly hinted that Kerry shouldn’t be receiving communion with a clear conscience, but so far, he hasn’t made it an official policy to deny communion to Kerry, or to any other pro-choice Catholic politician–therefore none of the churches in his diocese are obligated to deny Kerry communion.
What if O’Malley did make it an official policy? As bishop over Kerry’s diocese, he could do so, and then the churches of his diocese would be bound to follow that decision–the priests at Kerry’s church would then have to deny Kerry communion. Indeed, the archbishop of St. Louis, Raymond Burke, announced that he would not allow Kerry to receive communion at any Catholic church in St. Louis. So far as I know, Kerry did not test Burke’s pronouncement, but in any case, it only applied to the St. Louis diocese, and had no impact on Kerry’s diocese.
If O’Malley did make it an official policy to deny communion specifically to Kerry, and his church continued to give him communion, then the priests at Kerry’s church would be at odds with the bishop, and in some serious trouble. Kerry would face the same situation at any Catholic church in that diocese–but only in that diocese.
Needless to say, if Kerry were excommunicated, then he couldn’t receive communion or any other sacraments from any RC church in the world, and any priest that insisted on giving him communion would be in heresy with Rome.