Frankly, as someone who has participated in the temple ceremonies fryde67 addresses, the answer is quite simple.

1) The Presidency - or any other job or civic / professional responsibility - is not considered to be something a member "possesses." Of course, the personal morals a Mormon takes into a job or responsibility should affect the way s/he approaches and acts in that role (just like any other candidate's personal morals), but - as evidenced by Orrin Hatch and Harry Reid - the specifics of any member's actions will vary as widely as the political and religious opinions of multiple members. Many people don't realize this, but there are innumerable variations of beliefs within Mormonism (both political and spiritual) - much more so, for example, than in any evangelical congregation I observed when I lived in the Deep South.

2) I am of the same generation as Mitt Romney, and I have never - not once in my life - heard any Mormon pray for vengeance on America for the death of Joseph Smith, nor have I once heard anyone teach it - anywhere, inside the temple or in a regular church building or in a private home. It might happen somewhere in someone's home, but not one of the multiple thousands of members I have met in my decades as a member would approve.

3) The charge that I couldn't admit it even if I had been taught it is ludicrous - obviously coming from someone who doesn't understand the very limited restriction on what can and can't be shared from the temple ceremonies. There is a small booklet written by current Apostle, Boyd K. Packer (entitled "Temples") that can be read by anyone and describes almost everything that happens inside the temple. There is no quoting from the ceremony, but the essential elements are detailed quite extensively. Just so, I don't quote from the ceremonies, but there is almost nothing I won't discuss openly and fully.

On a personal note: My personal life is very conservative, but I am a political moderate. I was educated at a very prominent, very liberal college. Mormon theology actually is extremely liberal, but very few people understand that, since the stereotypical charges drone out reality. My congregation is as diverse politically as probably any religious congregation in this country. Congregations in Utah tends to be more predominantly Republican than those in Eastern states, and mine is quite representative of those outside of Utah. Most of the members I see weekly will not vote for Romney just because he is Mormon. This is an educated guess only, but I think he probably will get about 60% of the votes of those who vote in the Republican Primary - and about 40% of the members will not vote in that primary.

I would be happy to answer any specific questions from anyone, but I will not get involved in mud slinging and name calling and bickering. I can't check in constantly, but I will do so at least a couple of times daily.