If Mayor Pete is the nominee, his sexual orientation will be a big bullseye on his back in the general election. What would Trump do with it?
For all of his innumerable shortcomings, Trump really hasn’t shown much interest in bashing gays. It might also alienate moderate swing voters. I don’t think you would see any commercials from him on that subject.
That doesn’t mean his reactionary acolytes would follow his lead; in fact, he would probably count on that. The high road, low road approach with surrogates probably would work nicely here.
Otherwise, Mayor Pete doesn’t present a lot of obvious avenues for attack; he’s not a socialist, a woman, or an anti-war wimp. There is probably something in his tenure at South Bend that would work, and, of course, there is the McKinsey/elitist thing. For the most part, however, he would be a difficult target, which is a big argument in favor of nominating him.