There are lots of "most important Senate races," but to me, the most most important is the Indiana race between Democrat Joe Donnelly and extremist Republican Richard Mourdock. Why? Here's why.
Mourdock, as you know, beat longtime GOP Sen. Dick Lugar in a primary. We have important and sensitive negotiations coming up on Capitol Hill right after the election. If Obama wins, he'll need some Republicans to vote for a compromise on the fiscal cliff issues.
There are a handful who just maybe, under the right circumstances, might do so. But if Mourdock wins, no way. No Republican will compromise, because they will all fear being Mourdocked. Or Lugared, whichever you prefer to call it; it means that the non-extremist conservatives will be terrified of primary challenges from extremists in their next election, and none of them will do anything other than take the cowardly hard line.
Whereas if Mourdock is defeated, the reasonable conservatives will be able to say to their state parties that Mourdock-style extremism doesn't work and they need to win voters in the middle, hence a little bit of compromise is forgivable. The race is close, with dueling polls showing each in the lead. A great deal hangs on this one.