(CNN)Several Republican lawmakers have said it is too early to answer whether they'll back President Donald Trump in 2020.
Why a primary challenge to Trump is likely to fail
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%.
Now, it's not a perfectly linear relationship between approval ratings and primary difficulty for an incumbent president. The dividing line seems to be a 70% to 75% approval rating within your own party. Those above it do significantly better in primaries than those below it. It seems that when a president has an approval rating above the 70% to 75% line, prominent challengers are more likely to pass on the race.
Trump, of course, is well above the line right now. He actually has a higher approval rating among Republicans than Barack Obama did among Democrats just before the 2012 New Hampshire primary. That's probably why there aren't any potential challengers being named who really have too much of a future in the Republican Party.
The biggest name is Ohio Gov. John Kasich. Kasich could potentially make some hay in New Hampshire, though there's no reason right now to think he could actually threaten Trump's chance at the nomination. He only won his home state in 2016 primary season and struggled to win many votes outside of college-educated moderate voters in the northern part of the country.
Even if Trump never picks up the endorsements of prominent elected Republicans, I'm just not sure how much that will end up mattering. Trump had very few endorsements in 2016, and it didn't make a big difference in the end.
This time around, he'll likely have far more endorsements. South Carolina Sen. Lindsey Graham, for example, is already backing Trump, and they didn't exactly see eye-to-eye in the 2016 election.
Remember though it's still early, and it's going to be early for a while longer. Had people recognized the type of trouble George H.W. Bush would be facing in the 1992 election season, his most prominent challenger probably would have been someone with more star power than Pat Buchanan.
This time around, Trump could see his approval rating with his own party's voters fall for a number of reasons before 2020. The economy could go south. Voters might also lose faith in Trump if his party performs poorly in the midterm elections.
The far more likely path for Trump to be defeated in 2020 though is in the general election. Handicapping that race this early, however, is close to impossible.