It SHOULD be but history teaches us we cannot depend on this. If it were true, all Catholic lawmakers would be pro-life and in fact it is the Catholic lawmakers who are notoriously pro-abortion.
That is not a criticism of the Catholic church and I'm more than willing to use the example of that liberal Paul Simon as an example. I wouldn't have voted for him for dog catcher but yet he was a member of my church--the LCMS. I'll vote for a conservative Catholic, just about any religion as long as they have the same values as I.
The only religion I cannot tolerate is Muslim.
We cannot depend on the nominal affiliation, but we should be asking how serious the religion is ingrained in the politician. If there is no evidence of religion being taken seriously, then that is a character flaw that to me is worse than having a "wrong" religion. I would much rather have an anti-Catholic Protestant in the old mold than a wishy-washy Kerryesque pseudo-Catholicism.
The Muslim are of course in a category of its own since it is when they do take their religion seriously that we should see them as enemy. However, while I would not want Muslims anywhere near government, again, I respect the sincerity with which many Muslims take their faith.