We live in an age of party discipline. Even the most heterodox legislators – think Ben Nelson or Olympia Snowe – tend to fall in with their party when the chips are down. In this world, Republican legislators are Republicans and Democratic legislators are Democrats. What they personally believe is not a very good guide to how they generally vote. There are differences at the margins, when few are paying attention, but on major issues, when the country is watching, there is virtually no difference between the voting records of the fifth most liberal Republican and the fifth most conservative Republican.

We like to understand politicians as individuals and then we’re angry when their idiosyncrasies are overwhelmed by the demands of the system. But that’s not the fault of the individuals. It’s our fault, for voting and reporting and acting as if the system is irrelevant. The choice in Massachusetts is not between Brown and Coakley nearly so much as it is between a Democrat and a Republican. And all this goes double for someone who might entertain national ambitions, which I bet Brown will.