Where the argument falls apart for me is that, strictly speaking, it does not necessarily follow that because a politican supports laws that suffer women to have abortions under certain circumstances that one desires or encourages women to have abortions under those circumstances, or that one believes abortion is either virtuous or morally neutral. A similar argument could be made about the Marriage Amendment.
Many politicians, Catholics included, just don't think amendments to the Constitution are a good idea. That doesn't mean that they promote homosexual marriage.