Bob Corker's critical comments provoked a stream of Twitter insults from the US president
Donald Trump has intensified his war of words with Republican senator Bob Corker after the Tennessee politician again hit out at the US president's conduct in office.
Mr Corker, who has already announced he will not seek re-election to the Senate, has become one of the most high-profile critics of President Trump - and one of the few from within the Republican party itself.
In an interview on Good Morning America earlier today, he reiterated his view that the White House had become an 'adult day care center', and called on Trump to stay out of congressional tax reform efforts and "leave it to the professionals".
He later told ABC News that the president was 'debasing the nation'.
Mr Corker - who chairs the US Senate Foreign Relations Committee - explained: "I expressed concerns a few weeks ago about his leadership and his stability, and a lack of desire to be competent on issues and understand. Nothing has changed."
The Republican senator was a key Trump supporter during the presidential campaign - but he indicated that his opinion had changed since the business mogul had taken office.
Asked if he regretted supporting the then candidate, Mr Corker observed: "What do you think? There were many people - I was one of those - that hoped he would rise to the occasion as president and aspire to lead our nation instead of dividing it. It's obvious his political model and governing model is to divide. He has not risen to the occasion.
"It's a sad place from my perspective for our nation - and I think the worst of it is going to be the whole debasing of our nation [...] I don't think there's any question that that's the case. Just in the way he conducts himself and goes to such a low level."
In the wake of today's comments, President Trump again took to Twitter to slam his former GOP ally - describing him as "liddle' Bob Corker".
The US president wrote: "Bob Corker, who helped President [Obama] give us the bad Iran Deal & couldn't get elected dog catcher in Tennessee, is now fighting Tax Cuts.
"Corker dropped out of the race in Tennesse when I refused to endorse him, and now is only negative on anything Trump. Look at his record!"
Sen. Corker is the incompetent head of the Foreign Relations Committee, & look how poorly the U.S. has done. He doesn't have a clue as.....— Donald J. Trump (@realDonaldTrump) October 24, 2017
...the entire World WAS laughing and taking advantage of us. People like liddle' Bob Corker have set the U.S. way back. Now we move forward!— Donald J. Trump (@realDonaldTrump) October 24, 2017
However, Mr Corker - who had actually opposed the Iran nuclear deal, and whose office has claimed that President Trump asked the senator to reconsider his decision to retire - hit back at Mr Trump as an "utterly untruthful president".
Same untruths from an utterly untruthful president. #AlertTheDaycareStaff— Senator Bob Corker (@SenBobCorker) October 24, 2017
This afternoon, Mr Trump is having lunch with Republican senators - with the party's tax plans due to be discussed.