As Mr. Buttigieg spoke, Ms. Warren and Mr. Sanders were holding rallies in which they could scarcely utter two sentences without dropping in some formulation of the word “fight.” They spoke of the various “fights” they had led and the powerful moneyed interests they had “fought” and how they would “keep fighting” all the way to the White House.

Mr. Sanders touted himself as the candidate who would “fight to raise wages” and was “leading the fight to guarantee health care” and “fight against corporate greed.” Ms. Warren (fighting a cold) explained “why I got into this fight, will stay in this fight and why I am asking others to join the fight.”

Every politician wants to be known as a “fighter,” even the placid young mayor who has promised to “change the channel” on Mr. Trump’s reality show presidency and all the rancor that has accompanied it. But Mr. Buttigieg is also fighting against what he sees as the political trope of fighting per se. He is presenting himself as an antidote to the politics-as-brawl predilection that has become so central to the messaging of both parties and, he believes, has sapped the electorate of any hope for an alternative. “The whole country is exhausted by everyone being at each other’s throats,” Mr. Buttigieg said.