(CNN) Donald Trump was never part of the Washington in-crowd.

Yet the President is fast losing the few friends he did have in the capital, following a wild period in which he offered cover to white supremacists and ignited a war of words with North Korea, leaving GOP allies in the crossfire.

Thursday brought rebukes from two prominent Republican senators and a member of the highly influential Murdoch family, staunch supporters of the President.

Bob Corker, one of the most respected Senate Republicans, who has tried to keep open channels to the White House and coax Trump toward a more conventional foreign policy, unloaded on him in a spectacular manner.

The chairman of the Senate foreign relations committee is not prone to outspoken outbursts and thinks carefully before delivering his analysis. So his critique that the President has not shown sufficient stability, competence or understanding of the character of the country that he leads was devastating.

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South Carolina Sen. Tim Scott was another Trump ally who had been wrestling with a painful political and moral dilemma. But he also broke ranks Thursday, saying he could not defend the "indefensible" in the wake of Trump's comments about the alt-right rallies in Charlottesville.

Scott, the only black Republican senator, suggested that Trump had squandered the moral authority of his office -- a critical commodity vital in binding the nation together in a time of crisis or national tragedy that also helps to sustain the power of any presidency.

Another sign of Trump's growing isolation came Thursday night. James Murdoch, the 21st Century Fox CEO and son of Rupert Murdoch, who is one of Trump's close informal advisers, wrote a withering email denouncing the President's reaction to the White Supremacist rally and the violence it sparked.

"I can't even believe I have to write this: standing up to Nazis is essential; there are no good Nazis," Murdoch wrote. "Or Klansmen, or terrorists. Democrats, Republicans, and others must all agree on this, and it compromises nothing for them to do so."

Their sharp words were a clear indication of the damage that Trump inflicted on himself with his unchained news conference Tuesday, in which he drew an equivalence between racists and counterprotestors in Charlottesville and offered a shocking glimpse of his inner beliefs and temperament.

The defections of the two senators followed a stampede away from Trump by CEOs who served on White House advisory councils, amid fears their company brands could be tarnished by association with the President.

It all left Trump increasingly alone and even more reliant on his ever-loyal core voters, who sustain him in times of self-inflicted controversy and public outrage, but who may not represent a sufficiently broad base on which to build a successful presidency.

Corker and Scott have been slow to join the ranks of Trump's critics. Corker for instance, was on the President's short list to become vice president or secretary of state. And although he warned in May the White House was in a "downward spiral," he has not been a fixture on the chorus of Trump critics that includes Arizona Sens. John McCain and Jeff Flake.

Congressional tensions have a political price

The rebukes by Corker and Scott also pointed to a looming political problem that Trump has only exacerbated by his recent behavior.

The President was already lacking friends in the Senate, a reality that became clear in his narrowly failed push to repeal and replace Obamacare that left little doubt that few senators fear or revere him.

If he is to enact his ambitious political agenda that includes items such as tax reform and a massive infrastructure program -- or to influence Senate Republican legislation on these issues -- he will need to mend his toxic relations with top GOP members of the chamber, not least his feud with Senate Majority leader Mitch McConnell.

Every lawmaker is acutely conscious of the political pressures that weigh on them in their districts and states. The fact that Corker and Scott felt able to speak out may hint at trouble for Trump's political fortunes.

It may also reflect an ebbing of enthusiasm for the President among grassroots GOP voters. While 83% of Republicans approved of the job that the President was doing in the latest CNN poll last week, there were indications that the number of GOP voters who strongly approve of him is beginning to wane.

In a way it's not surprising that Republicans feel free to criticize the President. His presidency has been in crisis from almost its first day. His approval ratings are the lowest of any modern president at an equivalent stage, and his campaign is facing a special counsel probe into allegations of collusion with a foreign espionage service. Only the resilient economy that continues to pump out good job numbers may be preventing a complete free-fall.

The implications of Corker's remarks Thursday are staggering.

RELATED - Corker: Trump hasn't demonstrated the stability or competence to be successful

Taken to their logical conclusion, the remarks, from the lips of a friend, suggest that the President is simply not fit for office.

"The President has not yet been able to demonstrate the stability nor some of the competence that he needs to demonstrate in order to be successful," Corker said, according to a video posted by local news website Nooga.com.

The Scott and Corker critiques fit into a trend that is seeing Trump's circle of influence and political relationships shrink as his White House slips further into self-imposed isolation and crisis.

The now-disbanded advisory councils on which the CEOs served might have been little more than public relations efforts, but their demise dealt a blow to Trump, who relished gathering corporate titans to talk business and to stage photo-ops in which he starred.

In many ways, after a largely unproductive first seven months in office, Trump is again becoming what he has always been — an outsider.

His White House inner circle is dwindling, after the ouster of former chief of staff Reince Priebus and press secretary Sean Spicer, creatures of the Washington D.C. Republican establishment. There are rumors that another latecomer to Trump's campaign, his ideological alter ego Steve Bannon, may also be on the way out.

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This week, Trump, having failed to lure a big hitter to the West Wing, named Hope Hicks, one of his most loyal aides, as interim communications director.

In another sign of his insularity, the President -- even when he leaves the White House -- rarely spends time outside his comfort zone in Trump-branded properties. He has spent extended time at his resort at Mar-a-Lago in Florida and Bedminster in New Jersey. If he goes out in Washington, it's usually to the Trump hotel on Pennsylvania Avenue. If he's in the White House at weekends, he usually flees to his golf course on the banks of the Potomac in Virginia.

Testing the limits of an unorthodox strategy

Trump's narrowing political horizons are also reflected in his political strategy. In many ways, his unrestrained, unpredictable behavior of the last week recalls the persona that made him so popular with disaffected heartland voters.

By picking political fights on the status of Confederate monuments amid the fallout of his interventions on Charlottesville, he is raising issues that are important to a certain section of the conservative, Republican base.

Next week, the President will return to the embrace of his adoring core voters, at what is expected to be a raucous rally in Arizona.

His pitch is sure to position him as the anti-establishment champion of voters who revile Washington. So while the defections by senators like Corker and Scott are damaging to his hopes of getting things done, they actually offer a measure of validation in a different political context.

Many accounts of Trump's life and career stress that the President has never been the kind of person who cultivates vast networks of close political friendships. And he gives the impression that the only people who are not expendable to him are those in his family inner circle.

Right from the start of his rise as a brash, young, real estate up-and-comer from Queens, Trump was spurned by New York elites and seen as a brazen self-publicist. He was never part of the corporate crowd he courted as President.

But his experience of being spurned helps explains the uncanny connection he forged with working-class, heartland voters who felt excluded from the economic boons unleashed by globalization and the economic recovery after the 2008 crisis.

His instinctive understanding of his fellow outsiders also helped shape his ferocious assault on the GOP establishment — which made him President.

But it left him with a string of political enemies that made the friends he did have in Washington all the more important. That's why the rebukes from Corker and Scott may end up being even more damaging than they first appear.