"It's completely feasible that there will be long, drawn-out Republican primary for Trump to be renominated," Stutzman said.
Some of that may be wishful thinking by Stutzman, who has long been a vocal member of the "Never Trump" wing of the Republican Party. After all, there's no modern example of a primary challenge to a sitting president working. Ronald Reagan lost -- albeit barely -- to Gerald Ford in 1976. Ted Kennedy lost to Jimmy Carter in 1980. Pat Buchanan lost to George H.W. Bush in 1992. (Note: The primary challenges did not work, but none of the presidents listed above went on to re-election.)
At the same time, Trump may well be uniquely vulnerable to a primary challenge given his dismal approval ratings, his lack of connections to the major donors in the GOP and the fact that many Republican establishment types never wanted him to be their nominee in the first place.
In other words: A path does exist for a serious primary challenge to Trump if he stays on, roughly, the course he is currently on.
So, who might be the one to step forward to take Trump on? Here's a look at the five most likely contenders (listed in alphabetical order) :
Ted Cruz at a town hall in Austin, Texas, in July.
Sen. Jeff Flake walks to a Senate joint caucus meeting, on Capitol Hill.