Sen. Rob Portman (R-Ohio) announced Monday he will not run for a third term in the U.S. Senate in 2022, citing "partisan gridlock."

Why it matters: It's a surprise retirement from a prominent Senate Republican who easily won re-election in 2016 and was expected to do so again in 2022, creating an open Senate seat in a red-leaning swing state.

Between the lines: Portman was one of the Republican senators who said that former President Trump "bears some responsibility" for the Jan. 6 Capitol riots. His decision not to seek re-election will free him from the political constraints of voting to convict Trump in his upcoming impeachment trial, though it's not yet clear whether he will choose to do so.

What they're saying: "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," Portman said in a statement.

• “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."