Cassidy, a physician who has been a Louisiana senator since 2015, failed to advance in the party's primary for the fall midterm congressional elections. Two other Republicans are headed to a runoff this summer: State Treasurer John Fleming and Julia Letlow, who has received an outspoken endorsement from President Trump.
"I want to thank a special man whom you all know, the best president the country has ever had," Letlow said after the victory.
Cassidy, who has long been a thorn in the president's side, was restrained in his remarks after the loss.
"Our country is not about one individual. It is about the welfare of all Americans and about the Constitution," he said.
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