Unfortuantely, Mitch is a member of the GOP, and the only method of realizing the balancing power of the senate rules is with a solid, unified 40+ vote minority to block the road to tyranny.
You can rest assured that the enemies of the republic will always join together in a strong "coalition" party, and notwithstanding Washington's or Eisenhower's admonitions, you've GOT to have an opposition party for the designed-in gridlock to function.
So vote your conscience, but keep in mind that voting down-ticket is critical, regardless of how you vote for or against McPain.
Like it or not, if Obama wins, it will be called a landslide, he will be said to have a "mandate" for nearly anything shy of nationalizing industry, he will have large coattails in Congress, the popular perception will be that conservatism is dead, and there will be no political support for Congresscritters (like Mcconnel) to base their opposition to his policies or his Supreme Court appointees.
National Defense is one such issue, however I am of the mind that we can never again deploy our military into combat without the Constitutionally required "Declaration of War". Without it the "politicians" have to much wiggle room to squander the mission of war; Victory in the shortest time with the lowest cost in men, materiel, and money, by the earliest total destruction of our enemy!
We had that desire on 9/12/01, but failed to follow through.
I don't know what it will presently take, given the situation we find ourselves, in the Middle East, but somewhere down the line if we are to really "win" militarily Congress is going to have to send a Declaration of War to the President for his signature, which will put the entire Republic at war. Then an only then, IMHO, can and will we do what is necessary to WIN!