"His disloyalty to the man who got him elected is now a part of legend, and it's nice to see that his political career is OVER!" Trump wrote Saturday night on social media.

In backing Letlow against Cassidy, Trump achieved something he has not done yet in his extraordinary political career: defeat a senator from his own party in a primary. It came as he looks to unseat one of the biggest GOP mavericks in Congress -- Kentucky Rep. Thomas Massie -- in a House primary Tuesday.

Trump was quick to draw a connection between Cassidy and Massie. In a Truth Social post early Sunday morning, Trump said Massie "is an even bigger insult to our Nation than" Cassidy and reiterated support for Massie's primary challenger, Ed Gallrein.