Trump’s Louisiana Endorsement Shows How He Can Roil Midterms

President Donald Trump’s backing of a challenger to Sen. Bill Cassidy in Louisiana’s 2026 Republican Senate primary underscores Trump’s unpredictability that can upend campaigns.

Trump’s endorsement of Rep. Julia Letlow (R-La.) marks a break with Senate GOP leaders and is a body blow to Cassidy, a loyal conservative Republican who nonetheless antagonized Trump by voting to convict him in his second impeachment trial following the Jan. 6, 2021, Capitol riots.

The president’s opposition to Cassidy shows Trump’s prizes personal fealty above all and represents an unsettling move for Republican leaders who prefer to stick with incumbents.

“Especially if he takes ...