Senate Republicans reacted with alarm and dismay to President-elect Donald J. Trump’s decision to nominate Representative Matt Gaetz, Republican of Florida, for attorney general, and several said they were skeptical that he would be able to secure enough votes for confirmation. “He’s got his work really cut out for him,” Senator Joni Ernst, Republican of Iowa, said, chuckling as she spoke. Senator John Cornyn, Republican of Texas, raised his eyebrows when reporters informed him of Mr....