Sen. Mary Landrieu’s (D-La.) luck ran out Saturday night. The Democrat’s colleagues talk about her as a fighter who has won tough runoff elections in red Louisiana before, even if she was the underdog against Rep. Bill Cassidy (R-La.). But in 2014, a host of factors conspired to keep her from ultimately falling short for a fourth term. Collapse among white voters Landrieu’s support among black voters in Louisiana is nearly universal, but strategists in the state wondered if there were enough of them to counteract Cassidy’s huge lead among white voters. There were not. Landrieu’s percentage of the...