He said, in part: “I don’t think any Senate office has been more successful in getting things done, but honestly, it has gotten harder and harder to break through the partisan gridlock and make progress on substantive policy, and that has contributed to my decision.

“We live in an increasingly polarized country where members of both parties are being pushed further to the right and further to the left, and that means too few people who are actively looking to find common ground. This is not a new phenomenon, of course, but a problem that has gotten worse over the past few decades.”