FIRST ON FOX: Former Trump national security adviser Michael Flynn is endorsing former Missouri Gov. Eric Greitens in the Republican primary for the state's open U.S. Senate seat next year, bolstering Greitens' support from Trump world in a race that's become a battle to embrace the former president and his policies. 

"Worse than the radical leftists, the corrupt Deep State, the mainstream media, and Big Tech are the feckless and spineless Republicans who have utterly surrendered," Flynn says in his endorsement letter, first obtained by Fox News. "The people of Missouri recognize that those who betrayed President Trump the most were not the leftists but the cowardly Republicans in Name Only."

Flynn added: "That’s why I’m endorsing Governor Eric Greitens for the United States Senate."

Missouri Attorney General Eric Schmitt, the other high-profile candidate in the Missouri race, is also trying to associate himself with former President Trump – he touted his "fights" alongside Trump in an interview with Fox News earlier this year. 

Michael Flynn, President Trump's former national security adviser, leaves federal court in Washington, Tuesday, Sept. 10, 2019. Flynn is endorsing former Missouri Gov. Eric Greitens for the state's open U.S. Senate seat. (AP Photo/Manuel Balce Ceneta, file)

Michael Flynn, President Trump's former national security adviser, leaves federal court in Washington, Tuesday, Sept. 10, 2019. Flynn is endorsing former Missouri Gov. Eric Greitens for the state's open U.S. Senate seat. (AP Photo/Manuel Balce Ceneta, file)

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Greitens said in an interview Wednesday that he welcomes Flynn's support.

"I am so honored to have Gen. Flynn's endorsement," Greitens said. "He is a patriot. He is one of President Trump's strongest supporters and because of his leadership, because of his strength, because of his courage, he was attacked by the deep state and now patriots around the country are rallying to Gen. Flynn's side."

Greitens added: "The people of Missouri are behind me, and the people of Missouri are behind President Trump, because they recognize that those policies work for the people of Missouri."

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Among the other candidates for the seat – which is being vacated by retiring Sen. Roy Blunt, R-Mo. – are GOP Missouri Reps. Vicky Hartzler and Billy Long, as well as Mark McCloskey, who gained notoriety last year when he and his wife flashed firearms at a group of protesters they said were trespassing on their property.