I've written about the subject of Catholics in political life in general here and the Church's role in the public square here. Generally speaking, there is a signficant problem with "Catholic" politicians describing themselves as such while publicly opposing Catholic teaching on fundamental moral issues. The Church teaches that on most political issues there may be many valid positions to take - that is, religion would not compel a particular answer in, say, the question of whether California should get a grant to build a museum, or whether the maximum speed limit should be 55 or 70. On the opposite end of the spectrum, the Church does not (nor would it) require that Catholic politicians vote to enact theological precepts of their faith. But certain questions are more fundamental than tax subsidies and more universal (thus having a legitimate place in a democratic republic) than theological doctrine. The right to life, for instance, is the most basic one we have, and so all Catholic politicians are called to respect this right - as they speak publically and vote - when confronted with questions of abortion or euthanasia. The doctrinal note on participation of Catholics in public life speaks to this issue:
Living and acting in conformity with one’s own conscience on questions of politics is not slavish acceptance of positions alien to politics or some kind of confessionalism, but rather the way in which Christians offer their concrete contribution so that, through political life, society will become more just and more consistent with the dignity of the human person.
If John Kerry wants to quote Scripture, question the president's commitment to his faith, and tout his own religion, Kerry should try to make sure he himself is acting in conformity with - instead of flaunting - what his faith actually teaches according to that same Scripture.