Beguileya whiffs it on this point:
I love attack politics -- as long as it is fair, factual and about public issues. But even I am appalled by Donald Trump's insult-comic act. Don't stoop to his level.
In Iowa, especially, Democrats hate attacks. In a multicandidate field like this, it is highly likely that if Candidate A attacks Candidate B, Democrats will turn away from both A and B and suddenly Candidate C will surge. In the 2004 Iowa caucuses, Howard Dean and Dick Gephardt attacked each other. They each went from being at the head of the pack to losing to John Kerry by more than 20 points.
It had nothing to do with the bickering between Dean and Gephardt. Gep had no chance of performing to the polls run by the fevered minds of the Press (because he was a already a known loser), and Dean self-destructed when the voters of flyover Iowa got a load of his far-leftist act, and his pre-Antifa loony orange-hatted supporters.