This question has come up from time to time, with Mitt Romney in the race. My personal take on it is that there is no problem if Catholics want to vote for a Mormon. We are not voting for a national theologian, but rather a national political leader, and a candidate’s Mormonism is no more likely to turn him into a theocrat than is Catholicism or a mainline Protestant denomination.
That being my perspective, I was interested to read “Mormon Theology and Politics” by Michael Pakaluk in The Pilot. He also proposes that Catholics should not perceive Mormonism as a black mark against a political candidate, but he gives a more nuanced and reasoned-out argument. If the topic is of any interest to you, the article is worth reading.
That said, I am not necessarily endorsing any particular Mormon candidate; there may be other reasons not to vote for them.
Full disclosure: my father is a former Mormon from Salt Lake City, Utah, and my mother was briefly Mormon as well. Now they are both Roman Catholics. I was born Mormon, baptized Episcopalian, and raised Catholic…