It is perhaps not an accident that the most confident Democratic tribunes of good feeling are all men, while the party’s sternest warriors are mainly women. In a contest for the presidency, a position traditionally viewed in martial terms, it may be easier for a man of Mr. Biden’s backslapping swagger or Mr. Booker’s athletic stature to show tenderness or vulnerability without fear of appearing weak.

And it was with enthusiastic physicality, and regular references to having played high school and college football, that Mr. Booker preached love and understanding. He clasped his chest and his face at moments of emotion, usually stirring murmurs of appreciation and sympathy; in one case, he wrapped his arm around a voter for a midspeech selfie. While Mr. Booker said he was ready to spar with Mr. Trump, stating matter-of-factly that “there is nobody in this race tougher than me,” his overarching theme was about reconciliation.

At an airy adult learning center in Waterloo, Mr. Booker insisted that the capacity to conquer all manner of hardships was within human reach — a contrast with Ms. Warren and other populists, who tend to describe ordinary people being stripped of power by big institutions.