Anyone who concludes that the election of Scott Brown is a referendum on national politics is a liar or has never followed politics. No candidate is elected based upon that which he or she could-might-possibly do in the future. A voting person votes for the person with whom he or she most identifies. Candidate Brown was perceived as likable. Candidate Coakley, not so much.
Was she actually likable though? The confounding variable in the election is the Republican Machine™ which can easily transform an opposing candidate into a social outcast. Democrats get themselves elected when Republicans on the whole can be factually and demonstrably shown to be incompetent. Until an election forces the Democrats to distinguish themselves, they’re the left-wing of the Republican party.