Corker, who chairs the influential Senate Foreign Relations Committee and has emerged as an outspoken critic of the president, made the hard-hitting remarks after Trump attacked him in a series of morning tweets.
"I don't know why he lowers himself to such a low, low standard and debases our country in the way that he does, but he does," Corker told CNN, in a remarkable excoriation from within the commander-in-chief's own party.
"It's unfortunate that our nation finds itself in this place," he said, expressing regret for having supported Trump's presidential bid, and saying he would not vote for him again.
"He has proven himself unable to rise to the occasion" as president, Corker said in the interview conducted in a Senate hallway.
The fiery spat erupted just hours before Trump was due to attend a luncheon with Senate Republicans where he was expected to drum up support for a sweeping tax reform he wants to see adopted by year-end.
Corker, who has announced he is not seeking re-election in 2018, responded with a tweet of his own: "Same untruths from an utterly untruthful president."
After Corker's retort, Trump went on to brand him a "lightweight" and the "incompetent" foreign relations chairman who "doesn't have a clue."
The stinging verbal blows marked the latest salvo in what is now a full-blown public war of words between the president and one of his party's most important lieutenants in Congress, a measured Republican who has immense sway over foreign policy issues as well as taxes.
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