He probably won't win the primary. And even if he did win the primary, the Democrats are sure to win regardless in 2020. But I think this is a good sign that there is at least one person who is willing to primary Trump. Since he's so far removed from the GOP establishment at this point, he's got nothing to lose. I doubt any "real" GOP member will end up in the primaries, because they are too afraid they'll be shunned by the party afterwards.