Trump attacks retiring Republican Sen. Bob Corker in Sunday morning tweetstorm

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Sen. Bob Corker.

President Donald Trump on Saturday went after Tennessee Sen. Bob Corker, who recently announced that he would retire and not run for reelection next year.

"Senator Bob Corker "begged" me to endorse him for re-election in Tennessee," Trump wrote in a pair of tweets. "I said 'NO' and he dropped out (said he could not win without...my endorsement). He also wanted to be Secretary of State, I said 'NO THANKS.' He is also largely responsible for the horrendous Iran Deal!"

He added in another follow-up tweet: "Hence, I would fully expect Corker to be a negative voice and stand in the way of our great agenda. Didn't have the guts to run!"

Corker has been increasingly critical of the president and the GOP's agenda in recent weeks.

He made headlines earlier this week when he sharply criticized Trump for undermining Secretary of State Rex Tillerson's efforts to establish diplomatic channels with North Korea amid heightening nuclear tensions. Corker said Tillerson, Defense Secretary James Mattis, and White House chief of staff John Kelly are "those people that help separate our country from chaos."

And at a Senate Budget Committee hearing earlier, Corker was vocal about his concerns about the Republican tax plan that he worried could balloon the federal deficit.

"Unless it reduces deficits - let me say that one more time - unless it reduces deficits and does not add to deficits with reasonable and responsible growth models, and unless we can make it permanent, I don't have any interest in it," Corker said at the hearing.

Republicans hold a slim 52-vote majority in the Senate and can only afford to lose two votes to pass a tax overhaul, which is why Corker's vote is crucial.