Former acting attorney general Sally Yates, a Barack Obama appointee held over for the start of the new administration, could offer new details about how Trump handled concerns that national security advisor Michael Flynn lied about his ties with Russian officials.
She is expected to tell the Senate Judiciary Committee that she explicitly warned Trump's team in January that Flynn could be compromised by his communications with Russia's US ambassador.
Trump, who fired Yates soon after taking office, fired off a series of pre-emptive strikes ahead of her appearance on Capitol Hill, accusing Obama of enabling Flynn and assailing Yates for allegedly leaking classified information.
"General Flynn was given the highest security clearance by the Obama Administration," Trump tweeted early today.
More From This Section
"Ask Sally Yates, under oath, if she knows how classified information got into the newspapers soon after she explained it to W.H. Counsel," he added.
Yet despite those warnings, Trump named the former military intelligence chief his White House national security advisor, only to end up firing him after a month because Flynn had not been forthcoming about those calls.
Yates, known as a tough and independent prosecutor, has been a target of Trump's ire since he fired her at the end of January for refusing to support his controversial immigration ban on nationals from six Muslim majority nations.
"I will be asking the question that is are really on the minds of Americans," Democratic committee member Senator Richard Blumenthal said today ahead of the hearing.
"They want the truth uncovered about Russian interference and the last election and potential collusion of Trump associates," Blumenthal told CNN.
The focus on Russia's alleged "active measures" campaign against Western democracies has intensified following the leak of potentially damaging files from the campaign of Emmanuel Macron just days before he won France's presidential election yesterday.
Trump has repeatedly branded the issue "fake news" despite Clapper and the other leaders of the US intelligence community all having concluded that Russian President Vladimir Putin himself was behind the interference operation.
The Senate Judiciary Committee is one of several bodies in Congress probing Russian interference.
But progress has been slow amid Democratic accusations that Republicans are stalling the investigation to protect the White House.
Meanwhile, the Federal Bureau of Investigation has an active counter-intelligence probe into the scandal which is examining whether any Trump campaign official colluded with Moscow.