Sally Yates -- acting attorney general in the Trump administration for 10 days before being fired -- could bring new pressure on the White House over what it knew about former national security adviser Michael Flynn's communications with Russian officials.
Obama's director of national intelligence James Clapper is also set to testify, after repeatedly warning of the need to get to the bottom of how the Russians interfered in the election, and whether anyone on President Donald Trump's team colluded with Moscow.
The case simmered for weeks as attention focused on what keynote legislation the president could push through in his first 100 days, a milestone reached last week.
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Congressional investigations into Russian meddling have also been held up by infighting between Democrats and Republicans over how aggressively to pursue a matter that continues to cast a cloud over Trump's election win.
Dianne Feinstein, the top Democrat on the Senate Intelligence Committee, which also is investigating Russia's role in the election, said she is eager to hear Yates's testimony.
"Sally Yates is very much respected. She's a professional. She's not a politician. She's spent a lot of time in the department," Feinstein told NBC News's "Meet the Press" program.
"She apparently has some information as to who knew what when that she is willing to share - and that would be what she knew about Michael Flynn's connections to Russia," said Feinstein.
Trump: Russia story is 'phony'
Trump last week repeated his dismissal of US intelligence chiefs' conclusion that Moscow had sought to boost his campaign over his Democratic rival Hillary Clinton's in an effort overseen by Russian President Vladimir Putin.
In an interview with CBS's "Face the Nation" program marking his 100 days, Trump last week again rejected the official view that Russians hacked Democratic Party computers and communications.
"(It) could have been China, could have been a lot of different groups," he said.
On Tuesday, he again branded the whole story as fake. "The phony Trump/Russia story was an excuse used by the Democrats as justification for losing the election," he said on Twitter.
Trump's dismissals notwithstanding, the Senate Judiciary Committee - where Yates and Clapper are to appear on Monday - and the House and Senate intelligence committees are stepping up their probes, calling numerous current and former government witnesses to testify, mostly behind closed doors.
And the FBI is continuing its own investigation into possible collusion.
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The country's top intelligence officials say they have no doubt that Moscow tried to swing the election against Clinton last year through hacking and disinformation.
Nor do they doubt that people closely associated with the Trump campaign - including Flynn, onetime foreign affairs adviser Carter Page and former campaign chairman Paul Manafort - all had ongoing contacts with Russians.
But whether those contacts resulted in any collusion with Moscow remains unproved.
Asked on CNN last week if she had yet seen evidence of collusion in private intelligence briefings, Feinstein responded: "Not at this time."
In Monday's open hearing, Yates - an Obama deputy attorney general who was fired by Trump for refusing to support his immigration ban - reportedly could testify that she warned the incoming administration in January that Flynn's discussions with Russia's US ambassador left Flynn vulnerable to blackmail.
A former military intelligence chief, Flynn was Trump's national security adviser for 24 days before he was fired for lying about the substance of the calls.
Clapper, still bound by secrecy requirements of his former job, might not add more than what the intelligence community has already said publicly about the scandal.
Subpoena warnings
The more serious investigative action in the coming weeks will take place out of the public eye. The House and Senate intelligence committees are holding interviews with current intelligence and Trump campaign officials behind closed doors.
The Senate side has warned possible witnesses, including Flynn, Page and Manafort, that they could be subpoenaed to testify if they do not voluntarily cooperate, according to the New York Times.
In a statement Friday, the top senators of the Senate committee specifically warned Page, a former Moscow-based investment banker, to meet their week-old request for specific documents.
"Should Mr. Page choose to not provide the material requested" by specified dates, they said, "the committee will consider its next steps."
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