Well, sometimes I agree with Trump.
But I actually think running someone like Romney* as a third party Candidate helps Trump win. It’s all a question of who the votes come from. In a two way race there are people who will go Clinton over Trump and people who will go Trump over Clinton. Some of each group would really rather be voting for an independent right-wing candidate.
Romney>Clinton>Trump is a group composed of Republicans who just can’t stomach Trump and are going to support Clinton to save the country from a Trump nightmare.
Romney>Trump>Clinton is a group composed of Republicans who can stomach Trump but would have rather the party picked a different candidate.
Which of those groups sounds more likely to be able to get over the “taboo” of voting third party - the ones who stuck with the party even when it picked someone they didn’t like, or the ones who already jumped ship? Of course you could argue that R>T>C is just a bigger group than R>C>T so even if a small portion move it still benefits Clinton.
* Using “Romney” as a stand-in here, I’ve got my palm hovering inches from my face, just waiting in case it really is Romney.