Yates was thrust into the national spotlight after she broke with President Donald Trump over the enforcement of his travel ban, an action which led to her firing in January. But since then, her profile has only risen following revelations that she said she forcefully warned the administration about former national security adviser Michael Flynn's communications with a Russian diplomat weeks before Flynn was fired.
Just hours before Yates was scheduled to testify, former Obama officials confirmed to CNN that then-President Barack Obama warned Trump about hiring Flynn
as his national security adviser in their Oval Office meeting November 10. The news was first reported by NBC.
And CNN also reported
Monday morning that former Trump campaign adviser Carter Page told Senate investigators that he had "brief interactions" several years ago with a Russian official he said was a "junior attaché," even though US officials had suspected the official of spying on behalf of the Kremlin.
Here are five things to watch in Yates' testimony before the Senate Judiciary Subcommittee on Crime and Terrorism, which is scheduled to start at 2:30 p.m. ET on Capitol Hill.
1. What was the extent of Yates' warning?
Yates told White House staff and other top officials that Flynn could be a security risk after he discussed Russia sanctions with Russian ambassador to the US Sergey Kislyak, CNN reported last week
. That testimony is expected to contradict the White House's description of her warning, framed by press secretary Sean Spicer as "a 'heads-up' to us on some comments that may have seemed in conflict with what he (Flynn) had sent the vice president."
Trump on Twitter Monday blamed the Obama administration for giving Flynn, who briefly led the Defense Intelligence Agency during the 44th president's tenure, "the highest security clearance."
He quickly added: "Ask Sally Yates, under oath, if she knows how classified information got into the newspapers soon after she explained it to W.H. (counsel)."
But did Yates discuss more with the Trump administration? Were other interactions uncovered? Did she find additional information on other top Trump aides at the center of the Russia probes?
Her testimony comes after weekend reports in The Washington Post and CNN
that a former US official said Trump's transition team warned Flynn about the risks of communicating with Kislyak weeks before he did.
Flynn stepped down
from his position as national security adviser after reports surfaced that the Justice Department warned the Trump administration that Flynn misled administration officials regarding his communications with the Russian ambassador and was potentially vulnerable to blackmail by the Russians.
A White House official told CNN that the administration plans to rebut Yates by employing two strategies: Calling into question her objectivity by arguing she is a partisan Democrat and questioning the timeline of events she is expected to present, namely why "she and others at DOJ sat on their hands" after Vice President Mike Pence insisted on television Flynn had not discussed sanctions with Kislyak. Flynn's misleading of Pence before those interviews was a major reason he was forced to resign.
The official said that the White House views Yates as someone looking to launch a political career -- she has been rumored to be considering a bid for Georgia governor -- and that taking on the Trump administration could be the springboard she needs.
2. How limited will Yates be?
Democrats have pinned a lot of hopes on Yates, privately promising blockbuster testimony from the former Obama administration official. But a key question is how much she will be limited in public, either through questions regarding classified material, promises of attorney-client privilege or claims of executive privilege from either Trump or Obama.
Yates' first public hearing was scheduled for six weeks ago, but it was canceled at the last minute by Trump ally and House intelligence committee Chairman Devin Nunes. Reports later emerged that the White House blocked her testimony, which press secretary Spicer vehemently denied.
Monday's hearing comes a week after FBI Director James Comey spoke
to the full Judiciary Committee as well as House investigators. Comey, too, was limited in what he could say, citing an active investigation. As senators tried to pry any bit of information out of him, Comey beat back questions about the continuing federal probe
into Russian interference in the US elections, and he likewise revealed few new details to the House intelligence committee.
3. Did Yates' warning make it all the way to Trump?
We know now that Yates alerted White House counsel Don McGahn to Flynn's phone calls with Kislyak. Flynn's misleading comments, Yates said, made him potentially vulnerable to being compromised by Russia, according to sources familiar with her version of events. She expressed "serious concerns" to McGahn, making it clear -- without making a recommendation -- that Flynn could be fired.
What happened next isn't clear. After that meeting, the White House took three weeks to fire Flynn, which happened in February. But did McGahn ever tell Trump about Yates' warning?
4. Will there be anything left for House investigators?
The House intelligence committee was supposed to get the first crack at Yates in public, on March 28. But that hearing got swept up in the fiasco surrounding Nunes' clandestine White House trip and later led to him stepping aside from leading his panel's investigation.
It took a month for House investigators to even settle on inviting Yates back for a new hearing, but by that point Graham had already booked her for his investigation with the Senate Judiciary subcommittee. As of last Friday, House investigators still had not settled on a date for her new hearing or if there will be one at all.
5. It's not just Sally Yates testifying
Former Director of National Intelligence James Clapper is also set to testify. Look for questions about why his earlier assessment of Russian interference in the election contradicted the well-established version now, which is that not only was Russian interference omnipresent, but it was also designed specifically to help Trump.
Republican lawmakers have periodically brought this up over the last few months. Now will be their chance to hear directly from Clapper about whether his previous belief still stands.