American democracy is "under assault" by Donald Trump, the former director of national intelligence has warned.
James Clapper also said the sacking of FBI director James Comey was "another victory" for Russia.
Mr Clapper expressed dismay over the abrupt sacking of Mr Comey amid a probe into Moscow's meddling in the US election and its possible ties with members of the Trump campaign, which historians said was an "unprecedented act of presidential imperialism".
"I think, in many ways, our institutions are under assault, both externally — and that's the big news here, is the Russian interference in our election system," Mr Clapper said.
"I think as well our institutions are under assault internally."
When he was asked, "Internally, from the president?" Mr Clapper said: "Exactly."
He said America's founding fathers had created three co-equal branches of government with checks and balances, but that was now "eroding" with Mr Trump as President.
In a later interview with ABC News, Mr Clapper said "the Russians have to consider this [Mr Comey's sacking] as another victory on the scoreboard for them".
No White House aide appeared on the Sunday news shows, leaving Nikki Haley, the US ambassador to the United Nations, to defend Mr Trump.
"The President is the CEO of the country. He can hire and fire whoever he wants," she said.
Democrats and Republicans reprimanded Mr Trump for his actions, which included shifting explanations from the White House for Mr Comey's dismissal and an ominous tweet warning Mr Comey against leaks to the press because he may have "tapes" of their conversations.
They called for a new FBI director without any political background and said Mr Trump would need to hand over to Congress any taped conversations with Mr Comey, if they exist.
Less than a week after the President fired Mr Comey, the administration has interviewed at least eight candidates to be FBI director, and Mr Trump has said a decision could come before he leaves on Friday on his first overseas trip as president.
He abruptly fired Comey on Tuesday and later said Mr Comey was a "showboat" and "grandstander" who was not doing a good job.
Mr Trump said in an interview with NBC the Russia investigation factored into his decision.
Additional reporting by Associated Press
