Is a priest prevented from administering the Eucharist to a person the priest knows to be in mortal sin? a politician like Kennedy, for example?
A priest should be, if not by his bishop, by his conscience. That doesn't always happen. There's a little matter of free will and at this time in history, there are a number of revolutionary priests who are not hard line about this. i think there is some apathy and fear of making waves. There shouldn't be but no one ever said cowards couldn't be priests.
One of the bigger problems we're facing is some very weak bishops who do no teaching. This should be part of their teaching and when bishops like mine do say that politicians who are pro-death are not to take Eucharist (Burke was adament about it), they are labelled as extremist when all they are doing is teaching. Correctly, I might add.
A member of the clergy may refuse the Eucharist to a person who is known, by public action, to be living or advocating contrary to the teaching of the Church. The refusal is thus done to avoid scandal.
No one can know whether another person is actually in the state of mortal sin, and should not make judgements on that basis.
Canon Law 915 would almost require him to withould communion. That being said, I doubt one could find twenty priests willing to do that without first gaining diocesan support.