A group of influential liberal senators is directly challenging Sen. Chuck Schumer’s approach to the midterm elections and President Donald Trump, in another sign of growing frustration among Democrats toward the minority leader. The coalition of at least half a dozen senators, who call themselves the “Fight Club,” is unhappy with how Schumer and his fellow senator from New York, Kirsten Gillibrand, the head of Senate Democrats’ campaign arm, have chosen, recruited and, they argue, favored candidates aligned with the establishment. The mutinous mood of the senators — who include Chris Van Hollen of Maryland, Tina Smith of Minnesota, Chris...