Donald Trump has a lot riding on Arizona Senator Jeff Flake. Trump’s M.O. is classic bully: threaten someone mercilessly with the expectation that they will kowtow to you because you are stronger. Trump’s message to Flake is, “I can take your senate seat away by supporting your opponent so you’d better get in line.”
If Flake wins his primary battle a year from now (August 2018) then Trump’s threats will be shown to be empty. This could be right around the time that the Senate is considering impeachment charges against Trump, assuming the House actually proffers them. Even if not, senate committees could be uncovering a whole lot of unflattering information about the Donald, his business and his political alliances with the Russians, Arabs and others.
Not a good time to have Flake win the primary and show that your threats are harmless.