Booker’s campaign theme of universal love — a promise to transcend dispiriting partisan politics and the harsh tone of the Trump era — didn’t catch fire with voters on the left who have flocked to Sens. Bernie Sanders and Elizabeth Warren, who both promise a fight. More centrist voters have moved toward former Vice President Joe Biden and South Bend, Ind., Mayor Pete Buttigieg, who offer more moderate policies and may appear less risky to some voters who fear a black candidate would lose to President Trump.