Well, most people call them a primary even if.
The party itself calls it something different. Trump isn't the only person running for president under the republican banner. And if somehow he doesn't get the nomination, He will run as an independent and leave the republican party. And his supporters will vote for him, While all the other voters in the republican party vote for whoever the nominee is. Trump does not have enough support to win without Major party backing, And he would still enough votes away from the nominee.
That it would hand the election to biden.