(CNN) Several Republican lawmakers have said it is too early to answer whether they'll back President Donald Trump in 2020.

Yet barring some seismic event, Trump will most likely be their only viable choice.

Trump right now is adored by the Republican base. His favorability ratings among Republican voters are up significantly from before the 2016 election. His job approval rating (which is slightly different though tied to his favorability rating) in the latest monthly Gallup numbers is 85% among Republicans. Those numbers are quite strong and not predictive of a primary challenge.

I looked back at the approval rating just before the first-in-the-nation New Hampshire primary for every incumbent president who could run for reelection since 1952. Their approval rating with their own voters has been quite telling of whether they will have a significant primary challenge.

The five presidents -- George H.W. Bush, Jimmy Carter, Gerald Ford, Lyndon Johnson and Harry Truman -- who faced strong headwinds in the New Hampshire primary all had approval ratings among their own party members of less than 75%. The four presidents -- Carter, Ford, Johnson and Truman -- for whom the nomination was in doubt or dropped out of the race completely had approval ratings of less than 70%. The two presidents -- Johnson and Truman -- who dropped out completely had approval ratings of less than 55%.

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