Lt. Gov. Jay Dardenne (R) endorsed state Rep. John Bel Edwards (D) in a news conference at Louisiana State University, according to The New Orleans Times-Picayune.
"The Republican brand has been damaged," he said, as quoted by the Times-Picayune. "David Vitter will further damage that brand."
Dardenne's former campaign manager, Jay Vicknair, recently joined Vitter's runoff campaign. But losing Dardenne's endorsement is a blow to Vitter, who was trailing Edwards by 12 points in a Democratic poll taken Oct. 26-28. The runoff election is on Nov. 21.
This post has been updated.