Which senators might be vulnerable to a primary challenge in 2020?

Sens. Lamar Alexander and Pat Roberts, who both won their primaries despite pulling in a hair under 50 percent of the vote in 2014, benefited from multi-candidate fields that fragmented the vote to some degree and made it harder for the opposition to consolidate behind a single candidate. And in South Carolina, where a candidate must win at least 50 percent of the vote to win a primary, Lindsey Graham only avoided a runoff by a relatively slim 6-point margin. His night-and-day shift on President Trump may reflect his eagerness to survive another GOP primary.

Sens. John Cornyn and Mitch McConnell won somewhat more comfortably, but as party leaders — Cornyn is the Republican party whip and McConnell is the party’s Senate leader — both of them (and especially McConnell) bear the brunt of the public’s dissatisfaction with Congress. To differing degrees, all five appear interested in running again, so a tough slog to win renomination could be in the cards. And who knows which other races might become competitive — after all, Markey didn’t have an opponent in his 2014 primary, but he could be vulnerable this time around. Few could have foreseen Rep.-elect Ayanna Pressley’s win against Capuano in Massachusetts or Rep.-elect Alexandria Ocasio-Cortez’s victory over Rep. Joe Crowley in New York. In theory, Markey could be the victim of one of the next cycle’s primary upsets.