Robert Draper opens a window into it:

[Rep. Beto] O’Rourke went on to speculate, morosely, that [Rep. Chris] Van Hollen would not be the last person to exit the House Democratic caucus prematurely. “You look at my class, 2012,” he said. “Tulsi Gabbard, she’s not gonna stay in the House for long — she’ll run for governor or something else. Joe Kennedy, the same. Pat Murphy, the same. And they’re all talented, ambitious, good fund-raisers. But I’ve just got to think that when you see that 20-year road to being in a position of consequence, other options start to look a lot more attractive.”