Brian Stelter hit the nail on the head about the "big, uncomfortable story" surrounding Donald Trump's fitness and stability — or lack thereof — for the office he occupies.

When Tennessee Republican Sen. Bob Corker tweeted Sunday morning that it was “a shame the White House has become an adult day care center,” it was a moment of levity from an unexpected source.

But it was also a crucial and serious observation.

Corker’s tweet came after Donald Trump targeted him in his latest morning Twitter rant, insisting that “Bob Corker ‘begged’ me to endorse him for re-election in Tennessee. I said ‘NO’ and he dropped out.” Trump went on to claim that Corker said he couldn’t win without Trump’s seal of approval, and that his refusal to endorse Corker led him to choose not to run for re-election after all.

He further ranted that Corker had wanted to serve as Trump’s Secretary of State, and said he was “largely responsible for the horrendous Iran Deal!” And he whined that Corker’s “negative voice” against the Trump agenda was thus unsurprising.

The attacks arose from Corker’s public dissociation from Trump in recent weeks, from telling reporters that Trump “has not yet been able to demonstrate the stability nor some of the competence that he needs to demonstrate in order to be successful,” to declaring that key members of Trump’s Cabinet — like Chief of Staff John Kelly, Secretary of State Rex Tillerson, and Defense Secretary James Mattis — are the “people that help separate our country from chaos.”

And Corker, the Republican chair of the Senate Foreign Relations Committee, was indeed a supporter of the Iran nuclear deal achieved by President Barack Obama — and which Trump reportedly plans to “decertify” the deal and put the ball back in Congress’ court.

But whatever the basis for Trump’s Twitter rant, the fact of it is yet more evidence that the man currently occupying the Oval Office is so clearly unable to restrain himself or to exercise even a modicum of self-discipline when it comes to … well, nearly everything.

Attacking members of Congress is nothing new for Trump. In fact, attacking in general is not new for him, whether the target is an American mayor and her American constituents trying to survive a natural disaster; the media who won’t put his face on TV literally 24 hours a day; athletes exercising their First Amendment rights; foreign allies; immigrants; and on and on … and on.

Trump has either no understanding of or no concern for the traditional decorum of his office. And his own Cabinet members clearly believe their presence around him is necessary for the very safety of the nation.

This is utterly disturbing, and it is not normal.

And as CNN’s Brian Stelter put it succinctly, this — the manifest unfitness for office that Trump exhibits every day — is the biggest issue the country is faced with, and the thorniest one to confront.

We’ve come to expect President Trump to attack via Twitter, but to see Corker attack back, to use his ‘adult day care center line,’ is incredible. I would make the case that there’s no bigger story in America today than the fitness of the president of the United States. Is he fit for office? Is he stable for office? And that’s what Corker is getting at. He’s getting at that big, uncomfortable story about whether the president needs babysitting at the White House.

Now, we’ve heard people like Chief of Staff John Kelly privately, according to sources, privately saying that he is there as a chaperone, he is there to try to protect the country from President Trump. You look at the reporting from CNN on Friday, from Axios on Saturday — these men, like Kelly or Rex Tillerson — they’re in extraordinary situations right now, trying to protect Trump from himself, or protect the country from Trump.

Don’t take it from me — take it from those stories that have been written in recent days. We have the Secretary of State not denying that he called the president a moron. This is a huge story, and I think it’s notable because Corker is getting right at it by bringing up this day care center line.

Indeed, when one of the people closest to Trump reportedly labeled him an “effing moron” and yet remains by his side because of the worry of what could happen without his supervision, it is cause for serious concern.

Trump has never conveyed an iota of the seriousness or integrity the office of the presidency demands. Instead, he spends his time unleashing juvenile attacks on Twitter at anyone who dares to criticize him or even to fail to praise him to an acceptable degree.

Trump may have been well-suited to be the carnival-barker host of a reality TV show. But that is precisely why he is so patently unfit to be the president of the United States.