President Donald Trump and Sen. Bob Corker traded barbs on social media on Sunday.

The president claimed the Tennessee Republican, who announced last month that he would not seek re-election, begged for Trump’s endorsement before deciding not to run. Corker, for his part, implied that Trump is immature.

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What did the president say?

Trump said in a series of tweets on Sunday morning that he refused to give Corker an endorsement for what would have been his re-election campaign in 2018 and that Corker had hoped to be secretary of state.

“He could not win without my endorsement,” Trump wrote. “He also wanted to be Secretary of State, I said ‘NO THANKS.’”

The president said his refusal to back Corker would explain his “negative voice.”

“(He) didn’t have the guts to run!” he wrote.