That's kind of been my take all along, especially as he picked up steam. He really has them over a barrel now, but only to some extent. A Trump third party run probably puts Hillary in the White House, but it would also bring a batch of folks to the polls who would likely vote for conservative congressional candidates and maintain Senate control for the GOP. I don't see the House of Representatives going back to the Dems in my lifetime (I'm 58 and very healthy).
I've gone back-and-forth on Lindsay Graham over the years. I found him thoughtful during the Clinton impeachment hearings initially, but then he turned around and becomes a fire-breather within a space of weeks. He's kind of the same way with his foreign policy stance in that he lays it out logically and then basically says "What the heck, let's bomb them back to the Stone Age." I have never doubted his intelligence or sincerity, but like so many, he's got a missing piece (or three) that puts a ceiling on his career.