Senate Democrats doubled down on their opposition to the nomination of Sen. Jeff Sessions as attorney general, highlighting the need for independence in the office by pointing to President Trump’s Monday night dismissal of the acting attorney general. Praising the actions of acting Attorney General Sally Q. Yates, who Monday night defiantly announced her refusal to defend in court the president’s executive order banning refugees and travelers from some Muslim countries, Democrats questioned whether Mr. Sessions could be counted on to act independently of the president when warranted. Within hours of Ms. Yates’ announcement, Mr. Trump fired her, referring to...