Why would Mitt endorse a party that went out of its way to tell the rest of the country that Mitt is “not one of us”?
You’re the second person to post this.
I wonder why? AS I recall it, he had no values whatsoever that withstood a 24 hour news cycle. He would take a policy stance and change it on a dime. Not only that, he would deny he ever held some other opinion.
“If Hitler invaded Hell, I would make at least a favorable reference to the Devil in House of Commons.” - Winston Churchill, on Stalin.
Come on, it would be great if Cruz did it. And they’d cheer.
But really JohnT answered the question; George H.W. Bush would hand the election to Hillary Clinton if he spoke at the DNC endorsing her. Game, set and match.
Completely bizarre that the biggest “defector” to impact the DNC may very well be Donald J. Trump. 
Bunch of speechwriters earning their pay right this moment.
I’d like to see Pence burst into the convention seeking political asylum. He’s gotta wonder what he was thinking in agreeing to run with a sociopath.
Pence is just going to have to start repeating:
“Mr. Trump did not actually mean those words he spoke”
and
“I fundamentally disagree with everything that Mr. Trump has said.”
“Maybe I’ll get to be President.”
The simplest reason is probably true: Pence is a craven pile of dung.
Has anybody seen when Bloomberg is scheduled to speak?
So… no. No defectors of any note.
True, setting aside Bloomberg. As noted upthread former Reagan aide Doug Elmets and director of health policy at the U.S. Chamber of Commerce Jennifer Pierotti Lim were the two Republicans for Clinton.
Transcript of Elmets speech, FWIW: WTF?! What the Folly?!
Transcript of Lim’s speech: WTF?! What the Folly?!
You Tube of Lim’s speech: https://www.youtube.com/watch?v=lEpzyJbyris