Dan Drezner explains quite well why the midterms will not matter that much for foreign policy. Getting the START treaty reducing the number of American and Russian nuclear warheads was and is going to be difficult either way. However, as another blog on the foreign policy.com site discusses, the makeup of the Senate's Foreign Relations committee is likely to change. Yet another however: as the article discusses, this committee is the least significant of the major committees precisely because it does not make much in the way of $$ decisions, so payoffs for being on the committee are pretty limited. So, again, Dan is right--it just doesn't matter that much.
So, Dems can try to take some solace that it just does not matter: