The comments from Rep. Devin Nunes, a California Republican, came at the outset of a congressional hearing that will explore allegations of Russian hacking in last year's presidential election and whether there were any connections between Moscow and Trump's campaign.
FBI Director James Comey and National Security Agency Director Michael Rogers are to testify.
Trump, who recently accused President Barack Obama of wiretapping his New York skyscraper during the campaign, took to Twitter before the hearing began, accusing Democrats of making up allegations about his campaign associates' contact with Russia during the election. He said Congress and the FBI should be going after media leaks and maybe even Hillary Clinton instead.
"The real story that Congress, the FBI and others should be looking into is the leaking of Classified information. Must find leaker now!" Trump tweeted early Monday as news coverage on the Russia allegations dominated the morning's cable news.
Trump also suggested, without evidence, that Clinton's campaign was in contact with Russia and had possibly thwarted a federal investigation. U.S. intelligence officials have not publicly raised the possibility of contacts between the Clintons and Moscow. Officials investigating the matter have said they believe Moscow had hacked into Democrats' computers in a bid to help Trump's election bid.
Monday's hearing, one of several by congressional panels probing allegations of Russian meddling, could allow for the greatest public accounting to date of investigations that have shadowed the Trump administration in its first two months.
U.S. intelligence agencies concluded that Russia meddled in the campaign to help Trump defeat Clinton. The FBI has also been investigating ties between Russia and Trump advisers and associates during the campaign.
The top two lawmakers on the committee said Sunday that documents the Justice Department and FBI delivered late last week offered no evidence that the Obama administration had wiretapped Trump Tower, the president's New York City headquarters. But the panel's ranking Democrat said the material offered circumstantial evidence that American citizens colluded with Russians in Moscow's efforts to interfere in the presidential election.
"There was circumstantial evidence of collusion; there is direct evidence, I think, of deception," Rep. Adam Schiff, D-Calif., said on NBC's "Meet the Press." ''There's certainly enough for us to conduct an investigation."
Nunes said: "For the first time the American people, and all the political parties now, are paying attention to the threat that Russia poses."
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"We know that the Russians were trying to get involved in our campaign, like they have for many decades. They're also trying to get involved in campaigns around the globe and over in Europe," he said on "Fox News Sunday."
The Senate Intelligence Committee has scheduled a similar hearing for later in the month.
It is not clear how much new information will emerge Monday, and the hearing's open setting unquestionably puts Comey in a difficult situation if he's asked to discuss an ongoing investigation tied to the campaign of the president.
At a hearing in January, Comey refused to confirm or deny the existence of any investigation exploring possible connections between Trump associates and Russia, consistent with the FBI's longstanding policy of not publicly discussing its work. His appearances on Capitol Hill since then have occurred in classified settings, often with small groups of lawmakers, and he has made no public statements connected to the Trump campaign or Russia.
The Justice Department's disclosure Friday that it had complied with congressional demands for information regarding Trump's wiretapping tweets could allow Comey to avoid questioning by simply saying that the lawmakers already have the information they requested.
Yet any lack of detail from Comey will likely be contrasted with public comments he made last year when closing out an investigation into Clinton's email practices and then, shortly before Election Day, announcing that the probe would be revived following the discovery of additional emails.
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