Former Massachusetts governor - and 2016 Libertarian vice presidential nominee - Bill Weld will seek the 2020 Republican nomination. He becomes the first official challenger to President Trump. 

Weld had previously announced he was exploring a challenge to the incumbent president. As we noted at the time, "the history of serious incumbent primary challenges in the modern era is not a good one - either for the challenger or the sitting president. A strong primary challenge highlights fractures in a party, and often weakens the incumbent in the general election. We saw this most recently in 1992, where George H.W. Bush fended off Pat Buchanan, but lost the general election to Bill Clinton. Interestingly, that situation is somewhat the mirror of today. Trump represents the now-ascendant populist wing of the party, while someone like Weld would potentially appeal to the type of GOP championed by the Bushes."

Maryland Gov. Larry Hogan has also been considering entering the race. This article from fivethirtyeight discusses the difficult time he (or any challenger) would have against a president so popular with his own party.