The Watergate scandal of the 1970s is not as big as the Trump-Russia investigation, the former director of US national intelligence has said.
“Watergate pales” with what the US is confronting now, said James Clapper.
US intelligence agencies believe Russia interfered in the US election and they are investigating alleged links between the Trump campaign and Moscow.
But there is no known evidence of collusion and President Donald Trump has dismissed the story as “fake news”.
US intelligence chiefs are to be questioned on the matter by the Senate intelligence committee on Wednesday.
Senators say they plan to ask Director of National Intelligence Dan Coats, National Security Agency Director Admiral Mike Rogers, Acting FBI Director Andrew McCabe and Deputy Attorney General Rod Rosenstein whether Mr Trump attempted to derail the Russia inquiry by firing former FBI boss James Comey.
The testimony comes a day after the Washington Post reported that Mr Coats told his associates Mr Trump had tried to persuade the FBI to back off their investigation into his former national security adviser, Michael Flynn, and his ties to the Kremlin.
But Mr Coats, through a spokesman, said on Wednesday he never felt pressured by Mr Trump to end the investigation and suggested he would not reveal his discussions with the president.
The following day will see the much-anticipated testimony of Mr Comey, who was leading one of the Russia investigations before Mr Trump fired him.
He will be quizzed on his interactions with the president before he was sacked.
Mr Comey reportedly told Attorney General Jeff Sessions that he did not want to be left alone with the president.
Mr Clapper’s comparison with Watergate will raise eyebrows – President Richard Nixon resigned amid an unprecedented American political scandal involving spying, burglary and a cover-up.
The former intelligence chief, speaking in Australia, said it was “absolutely crucial” for the US – and the world – to get to the bottom of the allegations.