Sen. Bill Cassidy, a Louisiana Republican, has lost his bid for a third term in a remarkable defeat, after two primary challengers — including one backed by President Donald Trump — finished ahead of him in Saturday’s primary and headed to a runoff for his seat.

Senator Bill Cassidy of Louisiana on Saturday lost his Republican primary and the chance to seek a third term, after President Trump targeted him for defeat in retaliation for voting to convict him in...

Most polls suggest Cassidy is trailing Letlow and State Treasurer John Fleming, another Trump-aligned candidate, in the race.