I would say it was his mission. He is a liar for sure, but it is much easier and more accurate to say that he is a good salesman and does very well at selling himself and the conservative mindset to his constituents.
The sames skills of salesmanship work very well in the ranks of Mormonism.
I just finished watching the PBS Frontline documentary comparing the lives of President Obama and Mr. Romney. One thing that David Brooks and another former Romney strategist said some things that stuck out in my mind. Brooks said that Romney looks at politics as a marketing endeavor. The strategist noted that when they were contemplating the abortion switcheroo it didn't matter what was right or wrong, so much as what would sell best.
The LDS Church also seems to have a notion that the truth wouldn't sell well. I remember when I had a very serious conversation with my father about the reasons I was seriously weighing resigning from the LDS Church was that we taught things that were untrue to investigators as missionaries. He retorted that if we told them the whole truth they wouldn't join. That was a revealing comment. It showed me that to him it really didn't really matter to him whether or not the church was true, because he treated it like a kind of business enterprise to win converts to make the LDS Church as big and powerful as possible.
I think Romney sets objectives in a similar way. Right or wrong doesn't enter into it. You decide on a big goal and do whatever is necessary to achieve it. He thinks he'd be a fine President and what he does with that honored position doesn't really matter much, so long as he's the guy.