On Monday, in an open hearing before a Senate judiciary subcommittee, former acting attorney general Sally Yates gave her first public testimony in congressional investigations into possible Trump campaign collusion with Russian interference in the presidential election since President Trump fired her in January.

Yates told the Senate Judiciary Committee’s Subcommittee on Crime and Terrorism that on Jan. 26 and 27 she warned White House counsel Don McGahn that Lt. Gen. Michael Flynn, then Trump’s national security adviser, had engaged in “problematic” conduct and that he had misled Vice President Pence and others about it.

Although Yates said she could not discuss what that “problematic” conduct was because the information was classified, it is known from contemporaneous reporting by The Post that Flynn had misled Pence about the nature of his telephone calls with the Russian ambassador to the United States, Sergey Kislyak, during the transition period. According to media reports, law enforcement officials learned of the substance of the calls from routine intelligence intercepts of phone calls of foreign agents.

Yates’s testimony is the first public, on-the-record look at the chain of events that led to Flynn’s resignation on Feb. 13. It confirms that White House officials were, in fact, warned about that conduct more than two weeks before Flynn resigned — a resignation that happened only after The Post made it public that he had misled Trump administration officials, including Pence, about his communications with Kislyak.

Though the testimony from Yates — an Obama administration holdover acting in a temporary capacity while Trump’s nominee, Jeff Sessions, awaited Senate confirmation — was limited, it is significant. It confirms that McGahn was warned that Flynn had not been telling the truth about “underlying conduct” that Yates described as “problematic.” The vice president in particular, she said, had made statements based on Flynn’s deceptions “that we knew not to be the truth.”

The Russians, she added, “also knew about what Gen. Flynn had done, and the Russians also knew that Gen. Flynn had misled the vice president and others.” As a result, she went on, the Russians likely had information that created a situation “where the national security adviser could be blackmailed by the Russians.”

Yates testified that she talked with McGahn three times about Flynn, reiterating that “every time this lie was repeated, the misrepresentations were getting more and more specific,” and “every time that happened, it increased” the chances that Flynn would be compromised. “To state the obvious, you don’t want your national security adviser compromised by the Russians,” she concluded.

Yates’s testimony came as NBC News reported that former president Barack Obama warned Trump not to hire Flynn, who had been fired from his post as head of the Defense Intelligence Agency in 2014.

In advance of the hearing, Trump and his allies were clearly worried about what Yates would reveal. Axios’s Jonathan Swan reported this morning that White House officials planned to “brand Yates as a Democratic operative who was out to get Trump from the beginning and willing to torque the facts to advance her agenda.” (In fact, Yates, a 27-year DOJ veteran, has a lengthy résumé of prosecuting political corruption and white-collar crime.)

The backstory is crucial for understanding why Trumpland is so nervous about Yates.

On Jan. 30, less than two weeks after he took office, Trump abruptly fired Yates, ostensibly because of her refusal to defend legal challenges to his Muslim ban executive order.

But The Post’s subsequent reporting revealed that Yates was instead fired right after her warnings to McGahn. It also came to light that the Flynn-Kislyak communications were not innocuous small talk, as Flynn had suggested. About a week after Trump fired Yates, The Post’s David Ignatius reported that Flynn had called Kislyak “several times on Dec. 29, the day the Obama administration announced the expulsion of 35 Russian officials as well as other measures in retaliation for the hacking.” Nonetheless, Flynn publicly denied he had discussed sanctions with Kislyak.

The following week, The Post broke this key piece of the story: that five days before Trump fired her, Yates had warned McGahn about Flynn — which Yates’s testimony Monday confirmed. According to The Post, Yates, along with former director of national intelligence James Clapper and former CIA director John Brennan, agreed that McGahn should be warned that they feared “Flynn had put himself in a compromising position.”

Yet Trump, rather than addressing the Flynn situation, fired Yates. It wasn’t until two weeks later, and only after Pence learned from another report in The Post that Flynn had misled him, that Flynn resigned.

Today, Trump is bent on deflecting attention from the substance of Yates’s testimony by seeming to suggest, without any basis, that she has committed a crime. On Monday morning, Trump tweeted, “Ask Sally Yates, under oath, if she knows how classified information got into the newspapers soon after she explained it to W.H. Counsel.” Yates and Clapper both testified Monday that they did not know how the reports about Flynn made their way into The Post.

Trump’s tweet was reminiscent of his early March tweet in which he accused Obama of “tapp[ing]” his Trump Tower offices. More than two months since that tweet, Trump continues to maintain it has been “proven very strongly.” FBI Director James B. Comey has testified under oath that it is not true.

Yates’s testimony is particularly damning because it shows she clearly tried to warn the White House that Flynn had not only engaged in “problematic” conduct but also that he had misled Pence, who unwittingly then transmitted those deceptions to the American public. If Trumpland wanted to distance itself from Flynn and his conduct, why would it undermine Yates, rather than embrace her warnings?

The fact that the warnings weren’t heeded, and that it took public pressure to force Flynn out, makes that two-week period between the first Yates-McGahn meeting and Flynn’s resignation the crucial period for investigators to examine. And Yates, by confirming key details, makes this accounting even more urgent.