President Donald Trump's feud with a member of his own party erupted Tuesday, as the president delivered blow after blow on Twitter to Sen. Bob Corker, while the outgoing senator again warned about Trump's fitness to serve only hours before they both attended a Senate GOP policy lunch. In tweets Tuesday, Trump contended the Tennessee Republican is "fighting" tax cuts and "couldn't get elected dog catcher." The president again claimed that Corker chose not to run for re-election when Trump "refused" to endorse him and said he "now is only negative on anything Trump." Responding later in a CNN interview, Corker rebutted Trump and again took the remarkable step of questioning the president's fitness for office. The senator said "nothing" that Trump wrote in his tweets was truthful. He added that the president "debases our country" and will not "rise to the occasion as president." Corker said he would not support Trump again and contended that the president has "great difficulty with the truth," part of an escalating feud that complicates the GOP's push to overhaul the American tax system.

Sen. Bob Corker (R-TN) speaks to reporters in the Senate subway underneath the US Capitol, on October 17, 2017 in Washington, DC. Getty Images