It's not DeSantis that's dividing the party right now... 
It is entirely possible to have a primary where different candidates compete against each other for the nomination without "dividing the party". You have the primary, and then Republicans can together to support the eventual nominee. That was the norm before Trump.
But Trump doesn't want or believe in that. He wants to destroy politically anyone who dares compete against him in the primary, regardless of how much damage it does to party unity. And then he blames others for the damage
he causes.
It's essentially political blackmail. "Support me without opposition, or I'll divide the party and ensure a Democrat wins."