In Missouri, we elected a Democrat who ran on nothing. Absolutely nothing...except that our incumbent Republican Senator had sided with Bush. She won. The vast bulk of the population is located in St. Louis and Kansas City, in order for Republican to win the state, they have to win around 60% of the vote in the rest of the state. I don't think Romney would get that in Missouri. Too slick. Too east-coast. And Hillary will push her "Midwestern roots" from her days in Arkansas.
If there is not a strong turnout from Republicans, Hillary should win Missouri. I don't see Romney electrifying Republicans, even if I would vote for him.
I think Hillary would have a very hard time getting more than 45% of the vote in Missouri against Romney.