In a Republican-led Congress defined by deference to President Donald Trump, there's a small but steadily growing cohort who have found themselves more willing to break with the White House. Although the president maintains a firm grip on Republican voters, the expanding club could hinder his agenda on everything from the Iran war to immigration funding at a moment when his party holds a tenuous majority on Capitol Hill. Sen. Bill Cassidy of Louisiana is the newest member of the club. Just days after losing his primary to a Trump-backed challenger, Cassidy on Tuesday reversed himself on legislation involving the...