(CNN) Former acting Attorney General Sally Yates said Monday that she alerted the White House earlier this year that former Trump national security adviser Michael Flynn could be "essentially blackmailed by the Russians."

"We believed that Gen. Flynn was compromised with respect to the Russians," Yates told a Senate judiciary subcommittee, in a high-profile hearing on Russian meddling into the US election.

Yates told the panel that she had a meeting with White House Counsel Donald McGahn on January 26 to tell him that she had information that statements by Vice President Mike Pence, based on his conversations with Flynn, were false. She was joined in the meeting by a senior career official in the Justice Department.

"We weren't the only ones that knew all of this, that the Russians also knew about what General Flynn had done and the Russians also knew that General Flynn had misled the vice president and others," Yates said, relating the contents of her conversation with McGahn.

Yates was speaking at a hearing led by Sen. Lindsey Graham, who opened the hearing with an implicit rebuke of the President and his alternative explanations for the interference in the election.

The South Carolina Republican said the hacking was not the work of "some 400-pound guy sitting on a bed or any other country," a reference to a comment Trump has previously made on the matter.

Flynn lawyer Robert Kelner said Monday after the hearing's conclusion that he is declining comment on Yates' testimony.

Trump fired Flynn, a retired general, for failing to disclose discussions with Russian ambassador Sergey Kislyak about US sanctions against the Kremlin and for not telling the truth about them to Vice President Mike Pence. Yates did not say specifically that her concerns about Flynn's behavior was related to these calls, but she appeared to be implying that was the case.

In her opening statement, Yates said that she planned to be as "fulsome and comprehensive as possible" within ethical and legal boundaries.

Yates also warned in her opening testimony that there were some issues she could not address publicly because they involved classified information. Similarly, she said that as a former official she was not authorized to discuss Department of Justice or other executive branch deliberations. It was not immediately clear how those constraints would affect her testimony on the Flynn question. Neither Flynn nor Trump were directly referenced in her opening statement.

"The efforts by a foreign adversary to interfere and undermine our democratic processes — and those of our allies — pose a serious threat to all Americans," Yates said.

Graham asked Yates whether she had any information about whether there was collusion between members of the Trump campaign and Russia.

"My answer to that question would require me to reveal classified information," Yates said.

At one point in the hearing Graham asked both Clapper and Yates how information about Flynn's conversations with the Russian ambassador, that eventually led to his sacking, made it into the newspapers. Trump asked a similar question earlier on Twitter. Both former official said they did not know how that happened.

Trump went on the offensive on Twitter Monday morning, hours before the hearing began, blaming the Obama administration for Flynn's security clearance and asking the committee to question Yates over leaking classified information to the media.

"General Flynn was given the highest security clearance by the Obama Administration - but the Fake News seldom likes talking about that," Trump wrote , adding later , "Ask Sally Yates, under oath, if she knows how classified information got into the newspapers soon after she explained it to W.H. Council."

A White House official told CNN that the administration plans to rebut Yates by employing two strategies: Calling into question her objectivity by arguing she is a partisan Democrat and questioning the timeline of events she is expected to present.

The Senate judiciary committee's crime and terrorism subcommittee also heard from former Director of National Intelligence James Clapper, who spoke in advance of Yates. He testified that he did not know about the FBI investigation into Russian meddling in the election and whether there were any links to the Trump campaign until its existence was announced in a congressional hearing by FBI Director James Comey in March.

"During my tenure as DNI, it was my practice to defer to the FBI director -- both (former FBI) Director (Robert) Mueller and Director Comey -- on whether, when, and to what extent they would inform me about such investigations," Clapper said.

Trump hailed Clapper's comments and said they backed his claim that there was no evidence to support allegations of collusion between his campaign and the Russian government.

"Director Clapper reiterated what everybody, including the fake media already knows- there is "no evidence" of collusion w/ Russia and Trump," Trump said in the first of a series of tweets after the hearing.

Director Clapper reiterated what everybody, including the fake media already knows- there is "no evidence" of collusion w/ Russia and Trump. — Donald J. Trump (@realDonaldTrump) May 8, 2017

Likewise, a White House official drew attention to Clapper's comments and added: "Remember, the bottom line w/ the Russia stuff is the question of collusion during the campaign."

Clapper issued a clarion call for vigilance over Russian election interference before it further eroded US democracy.

"They must be congratulating themselves for having exceeded their wildest expectations," he said. "They are now emboldened to continue such activities in the future, both here and around the world, and to do so even more intensely."

Graham, the leader of Monday's hearing, also invited former President Barack Obama's national security adviser, Susan Rice, to testify with Yates and Clapper, but she rejected the invitation through her lawyer, noting the last-minute timing of the invitation. A source familiar with Rice's discussions told CNN that when Graham invited her, Rice believed it was a bipartisan overture and was prepared to accept. However, ranking Democrat Sheldon Whitehouse indicated to her that the invitation was made without his agreement, as he believed her presence was not relevant to the topic of the hearing, according to the source.

Of the four former Trump campaign aides at the center of the Capitol Hill's Russia probes -- including former campaign chairman Paul Manafort, former foreign policy adviser Carter Page and former campaign adviser Roger Stone -- Flynn has generated the most heat following a steady stream of revelations.

Investigators on the House oversight committee raised the possibility last month that Flynn may have broken the law by not disclosing payments from RT-TV, widely considered by US officials to be a propaganda arm of the Russian government, on his 2016 national security clearance form. Flynn's lawyer at the time argued that Flynn had been open about his speech to RT-TV, including briefing the Defense Intelligence Agency on his trip.

Yates' appearance itself had been fraught with drama ever since House intelligence chairman Devin Nunes' delayed her House hearing at the last minute, as part of a chaotic three-week stretch that saw the House Russia investigation almost fall apart and Nunes become the subject of a House ethics probe.

The Washington Post reported at the time that the White House had blocked Yates by asserting executive privilege, which allows the President to stop a former aide from testifying. White House press secretary Sean Spicer vehemently denied the reports at the time and said that the White House actively supports Yates' testifying in public.

A White House official said last week that the administration still wants Yates to testify in public and reaffirmed Spicer's comments. Graham and Whitehouse also said they heard of no effort to stop her from coming before them.

Still, Democrats on the House Russia investigation are anxiously watching Monday's hearing: First, to see if Yates shows up and, second, to see how much she reveals publicly.