The race for 2020 is taking shape, although there are still significant unknowns, including whether Donald Trump will get a serious primary challenge.
Trump's dominance of the party begins with his lockdown support of the right, forcing any primary challenger to the left. This isn't fertile territory. Self-identified moderates and liberals are only a fraction of the party, and it is grass-roots conservative activists who have fueled the most potent Republican primary challenges (Ronald Reagan in 1976, Pat Buchanan in 1992).
Because a primary challenge would naturally come from the left and is unlikely to succeed, it will tend to attract people who don’t have a future in GOP national politics and lack conservative bona fides — the wayward former Massachusetts governor Bill Weld; the centrist governor of Maryland Larry Hogan; the former Ohio governor John Kasich, who convincingly demonstrated his lack of national electoral appeal in 2016.