Who and when will be the first (serious) candidate to announce?

[Mr. Morden]

My bold prediction: Andrew Cuomo in February 2019.

Cuomo seems unlikely to announce the earliest, since he's up for reelection next year.  The candidates who are running for reelection next year will try to create the pretense that they're not sure if they're going to run for president, and then will wait a few months after their January swearing in before officially taking the plunge on a presidential run.  A pretend "I'm not sure yet if I'm running" period in early 2019 is probably more likely to happen for governors than for senators, since governors actually have responsibilities, whereas Senators can pretty much become absentee legislators whenever they want with little consequence.

So the candidates who will probably announce first are those who are unemployed, like Julian Castro and Martin O'Malley.  Then you'll get the sitting Senators who aren't up next year, like Booker and Harris.  Then you'll get any sitting members of either house of Congress who are reelected in 2018, then you'll get the governors.  Recall that Kasich and Walker didn't officially launch their 2016 campaigns until July 2015, making them the last to join the race out of the candidates who actually qualified for the debates.  And Christie and Jindal both announced in late June, so they were close to the end of the list as well.

[Mr. Morden]

I definitely think O'Malley. He is going to try and run as soon as you can so he can build up on his name recognition and be more of an established candidate when the rest of the field announces.

O'Malley's an interesting case, because I think he could actually get a tangible benefit from announcing early, which stems from the fact that he already has a decent level of name recognition in Iowa (at least compared to most of the field) from his 2016 run, and yet he's not really covered or even included in most of the polls right now, as the Senators are getting much more attention from the national media.

I don't actually believe that Iowa poll by O'Malley's PAC, which didn't even include Warren as an option.  However, in a fair poll of Iowa, with a realistic selection of candidates who might plausibly run, O'Malley making it into the top four on the basis of name recognition at least certainly seems *possible*, but that only works if you include O'Malley as an option in the polls, and most other polls so far don't.  So the benefit of announcing early would be that the media would notice him enough to at least include him in the polls, and then maybe just by showing up at 3rd or 4th place in a couple of polls in Iowa, the media starts talking about him, and he builds from there.  Though I guess this only works if candidates like Gillibrand and Harris don't grow their own name recognition at all between now and November 2018, or whenever O'Malley wants to get started.