From Jonathan Bernstein:
We have all become used to very boring presidential re-election campaigns. Bill Clinton in 1996 won easily; George W. Bush in 2004 and Barack Obama in 2012 made it close enough to induce some mild suspense, but not all that much, really. None of them had even a hint of a challenge for re-nomination.
That’s not going to be the case for Donald Trump, at least barring some extremely unlikely reversal.
In fact, the reporting over the weekend about early stirrings among potential Republican candidates for 2020 only confirmed what’s obvious: Unpopular presidents get nomination challenges. Here’s the history: