Look, if your point is about serving two masters, then you would have an objection to the Christian candidate who must serve God and his country - and you obviously don’t have a problem with that Christian candidate.
Your problem is not about serving two masters, it is about which master the candidate is serving - as you said, the LDS. THAT is your issue.
While I don’t agree with the teachings of the LDS, I don’t think there is any more danger of Romney serving the LDS ahead of or instead of the country or the Constitution than there is of Santorum or Gingrich serving the Pope ahead of the country or Constitution.
In this case it is the LDS. There is a difference between religions and a cult!
LDS is a cult (and I don't mean a sect...I mean a "cult"...which brainwashes its adherents)