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.
And the following day will see the much-anticipated testimony of the former FBI boss James 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 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.
In January, US intelligence agencies said Kremlin-backed hackers had broken into the email accounts of senior Democrats and released embarrassing ones in order to help Mr Trump defeat Hillary Clinton.
A report by the CIA, FBI and National Security Agency (NSA) said Russian leader Vladimir Putin "ordered" a campaign aimed at influencing the election.
Since then, Mr Trump has faced allegations that his campaign team had links to Russian officials.
Image copyrightGETTY IMAGESImage captionDevin Nunes (R), chairman of the House Intelligence Committee, with its top Democrat Adam Schiff
Republican Devin Nunes, chairman of the House intelligence committee, and Adam Schiff, the panel's top Democrat, are leading an investigation into the allegations.
Mr Nunes said on Sunday that based on "everything I have up to this morning" there is "no evidence" that the Trump campaign colluded with Russia.
Former Director of National Intelligence James Clapper has also said he saw no evidence of any collusion.
However, Mr Schiff said the material he had seen offers circumstantial evidence that US citizens collaborated with Russians to influence the vote.
"There was circumstantial evidence of collusion; there is direct evidence, I think, of deception," Mr Schiff said. "There's certainly enough for us to conduct an investigation."
Which campaign members have been accused of deception?
Two senior officials in the Trump administration have been caught up in the allegations - former national security adviser Michael Flynn, and Attorney-General Jeff Sessions.
Mr Flynn was fired last month after he misled the White House about his conversations with the Russian ambassador before he was appointed national security adviser.
Image copyrightAPImage captionMichael Flynn encouraged a softer policy on Russia and a harder line on Iran
He allegedly discussed US sanctions with ambassador Sergei Kislyak. It is illegal for private citizens to conduct US diplomacy.
Meanwhile, Mr Sessions was accused by Democrats of lying under oath during his confirmation hearing in January.
He said he had "no communications with the Russians", but it later emerged that he had met Mr Kislyak during the campaign.
Mr Sessions denied any wrongdoing, but removed himself from an FBI inquiry into Russia's alleged interference in the election.
What about Mr Trump's claims against Mr Obama?
Monday's hearing is also expected to address President Trump's claims that the Obama administration wiretapped his phone at Trump Tower in New York during the campaign.
Media captionTrump's wiretap saga explained in two minutes
Mr Trump has provided no evidence, and senior Republican and Democratic officials have dismissed the idea. Mr Obama's spokesman dismissed the claims.
Mr Nunes told Fox News on Sunday that a review of justice department documents provided on Friday indicated there was no such wiretap.
Several Republicans have said Mr Trump should apologise if he cannot substantiate his claims.
What impact have these claims had?
Observers say both allegations have diverted attention from the Trump administration's other policies and progress with political appointments.
Critics say Mr Trump's claim that Mr Obama wiretapped him has damaged the US credibility, and relations with its allies.
Image copyrightREUTERSImage captionMr Trump raised eyebrows after he suggested both he and German Chancellor Angela Merkel had been wiretapped by Mr Obama
Last week, Mr Trump's spokesman Sean Spicer repeated claims by a Fox News analyst that the UK's GCHQ spy agency had helped Mr Obama wiretap Mr Trump.
The claims angered the UK government, and GCHQ rejected the allegations as "utterly ridiculous".
Meanwhile, Mr Trump and some Republicans have called for an investigation into intelligence leaks, including the leak that revealed details of Mr Flynn's phone calls to the Russian ambassador.
