He might be the earliest candidate to launch his campaign, though.
Highly doubtful, IMHO. All of the candidates who face reelection in 2018 have to give themselves at least a few months into 2019 before officially launching their presidential campaigns, to at least keep up the pretense of not having already decided to run for prez when they were running to keep their current jobs. And in fact, that's probably more true for governors than Senators, since governors have real responsibilities, while Senators can slack off with few consequences.
I'd assume that the first people to announce will be those who no longer have a day job, like O'Malley and Castro.
