Lol I really want Cuban to primary him now. It's incredibly obvious he's scared of it. Cuban would wreck him in the debates.
Although Trump'd probably just skip them.
Yeah, I don't think any incumbent president has ever agreed to debate a primary challenger (though maybe I'm forgetting someone?), so I presume Trump would not agree to debate.
OTOH, if Trump gets two primary challengers rather than one, the two challengers (say, Cuban and Kasich) could have their own debate, and spend the whole hour ripping on Trump. Trump would complain and demand equal time, though the network could just tell him that he's welcome to show up for the next debate if he wants to rebut them.