Mr. Trump thinks Aesop’s fable of the Scorpion and the Frog doesn’t apply to him. He’s wrong. Romney will fatally sting him at the first opportunity.
I doubt he thinks Mitt won't try to sting him, Trump has been quite savvy at handling those who cross him. Ted Cruz is a perfect example. I don't think Romney is the one that's capable of giving Trump any kind of fatal sting, though he will likely try. Trump has shown himself to be a far superior person at political maneuver than Mitt.
I think this is a case of keeping his friends close, and his enemies closer. It's Trump's way of keeping the contagion contained. Unfortunately, Mitt is going to win, so keep him as close as possible.
Mr. Trump went to a studio and RECORDED that fable and released it to his adoring fans during the campaign as I remember. He knows what Mitt is. He knows him very well. In fact, if he said Mitt! Drop down on your knees, he’d drop down on his knees.