President Donald Trump’s drive for political revenge faces a key test Saturday in Louisiana, where he is looking to defeat Republican Sen. Bill Cassidy five years after Cassidy voted to convict him in his second impeachment.

Trump has backed Rep. Julia Letlow against Cassidy, though a second challenger — Louisiana State Treasurer John Fleming — has run a spirited race and made it likely that no candidate receives a majority of the vote, which would trigger a June 27 runoff.

It is a crucial time for Trump’s ability to show he can unseat fellow Republicans who cross him. The Louisiana election comes three days before Trump hopes to beat Kentucky Rep. Thomas Massie in his primary, a race that has drawn more attention.

Unseating a senator would be a new feat for Trump. While some of his intraparty detractors in the Senate have chosen not to seek reelection rather than face his wrath, he has never backed a primary challenger to a GOP senator who lost that primary. And it is rare for a Republican senator to lose renomination — the last time it happened was in 2017, when an appointed senator, Luther Strange of Alabama, was defeated in a primary runoff.

Trump reiterated his endorsement of Letlow in a social media post aimed at turning out voters for her Saturday, saying she is “a winner who will NEVER let you down.”