Donald Trump is “an utterly untruthful president” who is “debasing” the United States, a key Republican Senator said today in the latest round of a high-level political feud that has both captivated and appalled the country.

In a series of interviews and tweets, Senator Bob Corker, head of the US Foreign Relations Committee, repeated his belief that the White House was an “adult day-care centre” and that Mr Trump’s unpredictable behaviour was “taking us on a path to combat”.

“For young people to be watching, not only here in our country, but around the world, someone of this mentality as president of the United States is something that is I think debasing to our country,” Mr Corker said.

“You would think he would aspire to be the president of the United States and act like a president of the United States. But that’s just not going to be the case, apparently,” he added.

He wasn’t the only Republican senator to criticise Trump on what might eventually be seen as a watershed day in his presidency.

Just hours after Sen. Corker’s condemnation of Mr Trump, fellow Republican Sen. Jeff Flake of Arizona announced he would step down from the U.S. Senate at the end of his term, saying he was out of step with the Republican Party in the Trump era.

He would  “no longer be complicit or silent” in the face of the president’s “reckless, outrageous and undignified” behavior, Sen. Flake told the Senate in an occasionally emotional address.

“We must stop pretending that the degradation of our politics and the conduct of some in our executive branch are normal,” Sen. Flake said, adding: “They are not normal.”

Earlier, Sen. Corker gave a series of interviews in which he appealed to the President to stay out of discussions of tax reform and “leave it to the professionals”.

Predictably, Mr Trump responded with a lengthy series of insults on Twitter, saying in one that Mr Corker, who’ll retire at the end of his current term, “couldn’t get elected dog catcher in Tennessee”.

He also accused Mr Corker of engineering the Iran nuclear deal when, in fact, he opposed it.

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….

— Donald J. Trump (@realDonaldTrump) October 24, 2017

The senator responded on Twitter: “Same untruths from an utterly untruthful president. #AlertTheDaycarestaff”.

Same untruths from an utterly untruthful president. #AlertTheDaycareStaff

— Senator Bob Corker (@SenBobCorker) October 24, 2017

The public feud between the President and the senator broke out two weeks ago when Mr Corker accused Mr Trump of running the White House like it was an adult day care centre and putting America on the path to World War III.

Today, as the war of words grew more and more heated, Mr Corker was asked if the president was a liar.

“The President has great difficulty with the truth on many issues,” Mr Corker, who supported Mr Trump’s candidacy, told CNN.

Did he regret supporting him?

“Put it this way, I would not do that again,” Mr Corker said.

“He’s proven himself unable to rise to the occasion…he’s obviously not going to rise to the occasion as president.”

Mr Corker’s swipes clearly got under Mr Trump’s skin. He sent out a flurry of tweets variously accusing Sen Corker of being a “lightweight” and attacking him for his height.

“People like liddle’ Bob Corker have set the U.S. way back,” Mr Trump said in one of them.

Washington observers rejected Mr Trump’s attack on the senator’s status.

“POTUS may be the only official in Washington who considers Mr Bob Corker a lightweight,” Washington Post political correspondent Philip Rucker said in a tweet. “His reputation is that of a seasoned statesman.”

Ironically, the latest outbreak in the feud between Mr Trump and Mr Corker came just hours before the President was to lunch with Republican political leaders.

When Mr Trump arrived at the Capitol, a protester shouted “Trump is treason!” and threw Russian flags at the President.