Sally Yates. Pete Marovich/Getty Images) Former Deputy Attorney General Sally Yates will testify at a Senate Judiciary subcommittee hearing Monday about Russia's interference in the 2016 presidential election.

Yates, who was fired from her position as acting attorney general after refusing to enforce an executive order by President Donald Trump in January, was due to be interviewed by the Subcommittee on Crime and Terrorism on Monday about reports that she warned the White House in January about former national security adviser Michael Flynn's contact with Russia's ambassador to the US, Sergey Kislyak.

Yates is expected to publicly confirm those reports for the first time — and deliver a rebuke to the Trump administration in the process.

"Ask Sally Yates, under oath, if she knows how classified information got into the newspapers soon after she explained it to" the White House counsel, Trump tweeted early Monday morning.

Trump, who asked for Flynn's resignation after reports surfaced that he had misled Vice President Mike Pence about his conversations with Kislyak before Trump took office, has said he was not aware that Flynn had discussed the issue of US sanctions with the Russian ambassador.

According to CNN, however, Yates plans to insist that she told the White House about Flynn's conversations with Kislyak and how he was misleading Pence and was therefore susceptible to Russian blackmail.

While he was vice president-elect, Pence insisted in an interview with CBS that Flynn and Kislyak "did not discuss anything having to do with the United States' decision to expel diplomats or impose censure against Russia" — a statement that turned out to be untrue and that set off alarm bells at the Justice Department.

Michael Flynn. Win McNamee/Getty Images

Yates reportedly told White House Counsel Don McGahn that she had "serious concerns" about Flynn. But he was not asked to resign until February 13, at least two weeks after Yates visited the White House.

Interest in Yates' testimony grew even more last month after The Washington Post reported that the White House had tried to prevent her from testifying publicly. The White House has denied the charge, but it is unclear how much Yates will be able to publicly disclose because of restrictions imposed by the presidential communications privilege.

Former Director of National Intelligence James Clapper will also testify before the subcommittee, which is chaired by Republican Sen. Lindsey Graham — a Russia hawk who has said Flynn's request for immunity in exchange for testifying before the congressional intelligence committees about his Russia ties is "a bit bizarre."

“He's said in the past nobody asks for immunity unless they have committed a crime," Graham told CNN last month. "I'm not so sure that's true — as a lawyer I know that always that is not true. But the whole situation is really strange."

The House oversight committee's top-ranking Republican and Democrat — Reps. Jason Chaffetz and Elijah Cummings — said in a joint press conference on April 25 that Flynn appeared to have broken the law by not disclosing payments he received from Russia in his 2016 security clearance application.

"General Flynn was given the highest security clearance by the Obama Administration - but the Fake News seldom likes talking about that," Trump tweeted on Monday.

After news broke of Flynn's conversations with Kislyak, moreover, some questioned whether he may have violated the Logan Act — a 200-year-old law that bars private citizens from interfering with diplomatic relations between the US and foreign governments.

"The bottom line is if there were any contacts between the Trump campaign and the Russian intelligence services that were inappropriate, I want to find out about it, and I want the whole world to know about it," Graham said.

Clapper, meanwhile, told MSNBC's Chuck Todd in early March that he had seen "no evidence" of collusion between the Trump campaign and the Kremlin.

Clapper and Yates have also been invited to testify before the House Intelligence Committee about Russia's election interference in an open hearing this month. The hearing was originally scheduled for late March but was scrapped by the committee's chairman, Rep. Devin Nunes.

Nunes has since stepped aside from the committee's investigation into Trump's ties to Russia. He handed the investigation over to Rep. Mike Conaway, a Texas Republican, in early April amid questions about his ability to lead an unbiased investigation.