Watergate pales in comparison to the scandals engulfing Donald Trump and the Russia connection, former top US intelligence official James Clapper said Wednesday ahead of a key Senate hearing in Washington.
Clapper, who has worked in intelligence under every US leader from John F. Kennedy to Barack Obama, said his professional instincts had always been loyalty to the president, regardless of which party they come from.
But for Trump he is making an exception.
"Now as a private citizen, I am very concerned about the assault on our institutions coming from both an external source -- read Russia -- and an internal source, the president himself," he said in Australia.
Asked at the National Press Club in Canberra what he thought the critical differences were between Watergate, which brought down Richard Nixon, and Trump's troubles, he replied: "Watergate pales really in my view compared to what we're confronting now."
The US administration is at the centre of an ever-widening probe into Moscow's meddling in last year's election and possible collusion by the Trump team.
Clapper said he was at a loss to know why Trump's team appeared so keen to court Moscow.
"I characterise it as inexplicable. I don't understand that," said the director of national intelligence under Obama.
"It is absolutely crucial for the United States, and for that matter for the world, for this presidency, for the Republicans, for the Democrats and for our nation at large, that we get to the bottom of this.
"Is there a smoking gun with all the smoke? I don't know the answer to that."
Sacked FBI chief James Comey is set to testify Thursday as the star witness in the Senate Intelligence Committee's investigation of alleged interference and whether Trump pressured him to halt a probe into an advisor's links to Russia.
He is expected to dispute the president's claim that he told the former reality television star multiple times that he was not under investigation by the FBI.
"I think it will be very significant to see both what he says and what he is asked about and doesn't respond to," Clapper said of Comey, who he described as "a distinguished public servant".
Clapper said the "inexcusable manner" in which Comey was dismissed by Trump reflected "complete disregard for the independence and autonomy of the Federal Bureau of Investigation, our premiere law enforcement organisation".
"I worry about these assaults on our institutions."
He characterised Moscow's campaign to interfere with the US election as "unprecedented in its directness and aggressiveness".
"Social media trolls planting false information, orchestrated fake news which many other news outlets picked up and amplified either wittingly or unwittingly," he said.
"And a very sophisticated campaign by the regime-funded propaganda arm, the RT broadcasting arm, against Hillary Clinton and for Donald Trump.
"Their first objective, though, was to sow doubt, discontent and discord about our political system. They achieved, I'm sure, beyond their wildest expectations."