Maybe you don't see. Jesus also said "he who is not against us is for us." (Mark 9:40) Which principle to use obviously depends on the context. You seem to agree that Mormonism is opposed to true Christianity and since Mitt is a true believer, he would fit in the "he who isn't with me" rather than "he who is not against us". Maybe they are my categories but I believe they have Biblical support - sometimes neutrality is an issue, sometimes it's not. I'm still waiting for the compelling Biblical argument that it is okay to vote for an enemy of the Cross to such a high position of power and influence.
However, voting for president is not about who is a Christian and who is not.
I think that you're arguing in circles. I have already stated that I don't require theological purity in the candidates. That's what started the whole "enemies of the cross" discussion.
First, a vote for some other third party candidate at this point in history or a non-vote is a half-vote for Obama.
A non-vote for Romney is not a vote for Obama. As Revel points out, if that argument is true, I'm also giving Romney half a vote by withholding my vote from Obama, so they cancel out.
Second, a Republican president, however lacking in conservative principles, can be pressured by conservatives into doing the right thing.
You mean just like George W. Bush? Dubya was far more conservative than Romney. If the Republicans in Congress won't fight the unpopular President from the other party, what makes you think that they'll stand up to a President of their own party? The GOP-e is going to be pressuring the Republicans in Congress to go along with the President because "we can't afford to have a failed Republican Presidency." I'm very skeptical that Mitt Romney can be pulled right when his every instinct is liberal.
OK, this is pointless and tiresome. Daniel could loyally serve the unbelieving and even anti-believer kings of Babylon for decades, but a Christian cannot in good conscience choose the lesser of two evils between Romney and Obama. Paul could admonish Christians to “honor the king” when the king (emperor) was as pagan and anti-Christian as Romney, and probably more so, but a Christian cannot in good conscience choose between two men, one of whom openly identifies with the Constitution - imperfectly, to be sure! - and the other openly tries to subvert it.
Stay home. Don’t soil your pure hand in the voting booth.
Believe what you will about the valueless nature of your non-vote. The nation will no doubt benefit from the wisdom of your choice ... somehow ...