The race for the 2020 Democratic nomination is on. Who's in, who's out and who's a maybe.

For months, dozens of potential challengers to President Donald Trump have been “taking a look at,” “thinking about” and “not ruling out” a White House bid. The field, once crowded with potential would-be contenders, is now stacked with more than 20 declared candidates.

Here’s a timeline of the ever-expanding field, with slash marks denoting rumored candidates who took themselves out of the running.