■ Her testimony is likely to raise questions about how President Trump responded to concerns that his first national security adviser, Michael T. Flynn, had lied.

■ Ms. Yates can tell a dramatic story — a rarity in congressional hearings — but Democrats who hope she will reveal new information about the investigation into Mr. Trump’s campaign and Russia are likely to be disappointed.

What did the White House do?

Five days into the Trump administration, Ms. Yates alerted the White House to concerns about Mr. Flynn.

Reporters were asking whether Mr. Flynn had discussed sanctions with the Russian ambassador to the United States. The White House assured the public that they had not.

Ms. Yates, a temporary holdover from the administration of President Barack Obama, knew otherwise. That is because the United States routinely intercepts and transcribes the phone calls of foreign diplomats.

On Jan. 26, she told Donald F. McGahn II, the White House counsel, that the misstatements made Mr. Flynn vulnerable to foreign blackmail because Russian operatives would know that he had misled his bosses.

Details of that conversation, including Mr. McGahn’s response, remain unknown, and Ms. Yates can shed light on what was said. Officials have said that Ms. Yates told Mr. McGahn how to obtain and read the call transcript himself.

Ms. Yates’s account could put pressure on the White House to more fully explain its response. The president  https://www.nytimes.com/2017/02/13/us/politics/donald-trump-national-security-adviser-michael-flynn.html" rel="nofollow - ultimately fired Mr. Flynn , but not because of Ms. Yates’s warnings. Mr. Trump acted two weeks later, only after The Washington Post  https://www.washingtonpost.com/world/national-security/justice-department-warned-white-house-that-flynn-could-be-vulnerable-to-russian-blackmail-officials-say/2017/02/13/fc5dab88-f228-11e6-8d72-263470bf0401_story.html?utm_term=.4049b304874e" rel="nofollow - found out about those warnings .

Obama warned Trump about Flynn, officials say.

Mr. Obama warned Mr. Trump against hiring Mr. Flynn when the two met in the Oval Office two days after Mr. Trump was elected, two former Obama administration officials said Monday.

Mr. Obama, who had fired Mr. Flynn as the head of the Defense Intelligence Agency, told Mr. Trump that he would have profound concerns about Mr. Flynn becoming a top national security aide, said the administration officials, who were briefed on the Oval Office conversation. Mr. Trump ignored the advice, naming Mr. Flynn to be his national security adviser.

Photo

Sally Q. Yates at a Senate Judiciary Committee hearing in 2015 on her nomination as acting attorney general.CreditPablo Martinez Monsivais/Associated Press

Mr. Flynn was forced out after it was revealed that he had lied to Vice President Mike Pence about his conversations with the Russian ambassador, but Mr. Trump seemed to blame the Obama administration on Monday for the problems surrounding Mr. Flynn.

It was not entirely clear what the president meant. Mr. Flynn’s security clearance was not an issue in his firing. Mr. Trump said he fired him for lying to Mr. Pence.

A presidential Twitter preview.

The hearing was clearly on Mr. Trump’s mind hours before it started.

In addition to his tweet about Mr. Flynn’s security clearance, Mr. Trump also suggested on Twitter that Ms. Yates had tipped off journalists about Mr. Flynn’s conversations with the Russian ambassador. That is a familiar beat for Mr. Trump, who has said repeatedly that the leaks of classified information are far more significant than the actual connections between Russian officials and the Trump campaign. (“The leaks are absolutely real,” he said  https://www.nytimes.com/2017/02/16/us/politics/donald-trump-press-conference-transcript.html" rel="nofollow - at a news conference  in February. “The news is fake.”)

A storytelling moment.

Senate subcommittees are rarely the setting for high drama.

But Ms. Yates has an untold story, and senators — particularly Democrats — have an incentive to dial back the usual long-speech-short-question format and let her tell it.

Ms. Yates would not normally be allowed to testify about her conversations with White House lawyers because such discussions are typically considered privileged. But White House officials have discussed the conversations publicly, which gives Ms. Yates more leeway in what she can say.

Still, telling her story will not be easy because, even though it is widely known that the United States eavesdrops on foreign officials, the existence of a wiretap on the Russian ambassador remains classified. So Ms. Yates is not likely to be allowed to say what made her concerned about Mr. Flynn’s actions.

Russian meddling.

Senators Lindsey Graham, Republican of South Carolina, and Sheldon Whitehouse, Democrat of Rhode Island, are leading the hearing into Russian interference in the presidential election.

“On Monday, Senator Graham and I will hear from federal officials who can speak to the hard facts of Russia’s meddling in our election. We will pose a range of questions about the tools Russia used, which we learned about in our first hearing, and help to establish for the American people what happened and how to guard against it moving forward.” — Sheldon Whitehouse

Ms. Yates, who was deputy attorney general during the last year of the Obama administration, can talk about Russian meddling and the government’s public conclusions. But she is expected to sidestep questions about the Justice Department’s investigation into the Trump campaign and possible collusion with Russia.

Clapper was unaware of Trump investigation claims.

After Mr. Trump accused Mr. Obama of wiretapping him during the campaign — a universally rejected accusation  https://www.nytimes.com/2017/03/16/us/politics/richard-burr-mark-warner-trump-wiretap.html" rel="nofollow - for which there remains no evidence  — Mr. Clapper, the former director of national intelligence, went on NBC’s “Meet the Press” and  http://www.nbcnews.com/politics/politics-news/former-dni-james-clapper-i-can-deny-wiretap-trump-tower-n729261" rel="nofollow - made a sweeping denial .

Journalists have  https://www.nytimes.com/2017/04/12/us/politics/carter-page-fisa-warrant-russia-trump.html" rel="nofollow - since revealed a wrinkle in that story . The F.B.I. obtained a court-approved wiretap on Carter Page, a foreign policy adviser to Mr. Trump’s campaign, based on evidence that he was operating as a Russian agent.“There was no such wiretap activity mounted against the president, the president-elect at the time, or as a candidate, or against his campaign,” Mr. Clapper said.

Mr. Clapper testified Monday that he was unaware of the F.B.I. investigation into the Trump campaign, “and that comports with my public statements.” That secrecy shows how closely the F.B.I. guarded its investigation into Mr. Trump last year.

Former government officials have said that Mr. Clapper, even though he was unaware of the investigation, was correct in saying that neither Mr. Trump nor his campaign was wiretapped. The Foreign Intelligence Surveillance Court issued the warrant only after Mr. Page was no longer part of the Trump campaign. But senators are likely to press Mr. Clapper on that point.