Letlow, who became the first Republican woman elected to represent Louisiana in Congress in 2021, won about 45% of the vote in the primary, US media report.
Fleming, her rival in the June primary runoff, previously was a representative for Louisiana in Congress and also worked in the first Trump administration.
He narrowly beat Cassidy with around 28% of the vote, compared to Cassidy’s 25%, according to US media.
“Yesterday, the people of Louisiana proved that a grassroots conservative can still compete, and win, even when the political establishment and outside dark money groups spend millions of dollars trying to destroy him,” Fleming said in a statement on Sunday.
