Democrat John K. Delaney, who represents Maryland's 6th congressional district, has become the first formally declared challenger to a second term for President Trump.  Delaney, who is in his third term, will not run for Congress in 2018 so that he can focus on the presidential bid. Delaney laid out his rationale for running in a Washington Post opinion piece and has launched his campaign website that includes this video:

There are just over 1,190 days until the 2020 presidential election, making Delaney's announcement the earliest one in recent history (probably longer). Delaney is not well-known nationally, so the novelty of going first will garner him some attention.

We've added Delaney as a candidate option (click or tap 'Democrat' above the electoral counter) on the 2020 interactive map.