Donald J. Trump, who is rarely at a loss for words, admitted “I don’t know what’s going on” when confronted by Ben Carson’s surge past him in early-voting Iowa, where Mr. Trump had led the Republican presidential field for months. Mr. Trump has derided Mr. Carson for lacking the vigor and fortitude to be president, but voters here are drawn to the retired neurosurgeon’s low-pitched manner. “That smile and his soft voice makes people very comforted,” said Miriam Greenfield, a farmer in Jewell, Iowa.