A practicing Catholic as an individual, yes. A practicing Catholic as a representative of all the people, no, or at least not if the Church wants to operate in a pluralistic society. If the Church believes that their moral doctrines should be enacted, what is their defense if a Moslem representative wants to ban the use of alcohol everywhere, including during Communion?
I agree with you somewhat on what the politician should do, but I also think the church should back off. Actually Kerry’s position on abortion - I’m personally against it, but I’m not going to impose my religious beliefs on others - is exactly right.