Trump arrived on Capitol Hill to attend the Senate Republican caucus lunch for the first time since his inauguration, to rally support for passing a tax overhaul before year's end.
But attention instead has swerved to a brutal back-and-forth with Senator Bob Corker, who charged in response to a Twitter attack from the president that Trump is an "utterly untruthful" leader who "debases" the nation.
Corker, an influential Republican who chairs the Senate Foreign Relations Committee and has emerged as an outspoken critic of the president, excoriated him earlier this month as dangerously impulsive and branded the White House an "adult day care center."
He appeared to further antagonize Trump when he told ABC early Tuesday that he would like the president to stand clear of the debate over tax legislation and "leave it to the professionals" to finalize the plan.
Trump rounded on Corker in a series of tweets, and the war of words was on, playing out on television and social media in real time.
"Bob Corker, who helped President O give us the bad Iran Deal & couldn't get elected dog catcher in Tennessee, is now fighting Tax Cuts," Trump tweeted.
"Same untruths from an utterly untruthful president," shot back Corker.
"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 went on to tell reporters at the Senate. "He has proven himself unable to rise to the occasion."
Corker -- who himself is not running for re-election next year -- expressed regret for supporting Trump's presidential bid, and said he would not vote for him again.
'Doesn't have a clue'
The rapidly escalating feud has laid bare the tensions between Trump and some in his party on Capitol Hill, where the president has yet to grab a major legislative victory.
Trump's tax reform push is his latest attempt to do so.
Speaking in the Oval Office mid-day Tuesday, Trump said the United States is one of the highest-taxed countries in the world and a tax cut would, as he put it, restore America's competitive edge.
"If we get this fast, which I really believe we will because we have to as a country, it's going to bring back, I would say, four trillion dollars back into this country," Trump said.
He added: "That money will flow back in, it will be very quick and it will be very easy."
Inside Congress, as the president made his way to the Republican luncheon, a protester yelling "Trump is Treason" hurled small Russian flags into his path, accusing him of conspiring with Moscow to tilt the election his way.
In the past week Trump has been harshly criticized by two high-profile Republicans: former president George W. Bush, who said "bigotry seems emboldened" in the Trump era, and Senator John McCain, a former war hero and 2000 presidential nominee, who issued a searing rebuke to Trump's ideas and politics.
Neither Bush nor McCain mentioned Trump by name, but the target of their criticism was clear.
After Corker's diatribe, Trump pushed back on Twitter, branding him a "lightweight" and an "incompetent" foreign relations chairman who "doesn't have a clue."
"People like liddle' Bob Corker have set the U.S. way back. Now we move forward!" Trump added.
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.
The speaker of the House, Paul Ryan, sought to downplay the row, reminding reporters of the importance of staying focused on "a historic chance of actually fixing this tax code."
"So all this stuff you see on a daily basis, Twitter this and that, forget about it," Ryan said.
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