With the demographics of the state, I think Democrats are always competitive. They probably start at about 40% and they just have to pick up the rest. I think even Holden would be competitive. Blunt will have to fight hard.
By the same token, Republicans start with slightly more than 40% in Missouri. Margins tend to be 51% to 49% or even closer, but Republicans win the 51% about 60% of the time - we've had a few more GOP senators and governors than Dems for the last couple of decades. This moribund old state changes only very slowly, but is trending just slightly more GOP over the years.
I actually haven't been in the US for 5 months or so, so I can't report on the mood in MO directly, but all that I hear is that Holden is unpopular - certainly he was barely elected the first time. Its going to come down to another typical landslide for the Democrat in St. Louis and a landslide for the Republican in the rural and suburban parts of the state, balancing out to neary 50/50. It could easily be influenced by Bush getting (I predict) a breakout, for Missouri, 52-53%. The Blunt family name is well known in the state, certainly positive in the non-urban parts.