The republican party doesn't have to have to be Trump, but only once in history has an elected president not been his parties presidential candidate after their first term; so Trump not being the republican candidate for 2020 would be pretty remarkable, having not happened since 1850s.
However, Trump has motivated the Democratic base, getting more independents or usually non-voting individuals out to vote to oppose him. He also has seemed to alienate a number of republican congressmen and generally been somewhat divisive within the republican party itself. However, his popularity numbers with republican voters is a little low, but not horrible so, suggesting the republican's voting base may be willing to re-elect him in primaries.
It's too early to say for certain what will happen, but I'm wondering what the general odds are that Trump may not be the republican candidate of 2020? What factors may play into deciding rather he is elected as the republican canidate?