The external political world that sends senators to Washington has changed. The greater political world is more polarized then it was a few decades ago and the two parties are now aligned based on their ideologies. The problem with the Senate is not that senators are a mediocre bunch or that they are more partisan then in the past. The problem with the Senate is the world outside has changed and the Senate has not adjusted it's internal rules and norms to reflect that change.
12 minutes ago