It’s not unusual for a 65-year-old to announce his retirement, but a lot of people were caught by surprise when Sen. Bob Corker of Tennessee did so on Tuesday. In a statement, the senator, who has found himself increasingly at odds with President Trump, said he he hadn’t planned to serve more than two terms in office. But is there more at work in the departure of this influential senator than simple adherence to a self-imposed term limit? What does it portend for the Republican Party establishment, Trump and the future of the party? There are a few ways to think about it.