Respectfully to tom w, the resistance of copper wires (all conductors of electricity do this) actually falls the colder it gets. And, you could have a failing starter, but check the grounds (and the battery connections).

I do think tom is right about the grounds and you should probably take off and clean all ground connections and the connections at the starter. They can be looser and fail with lower temps. Also, according to my MB manual, a good battery has 65% of it's current output at 32F versus 70F.