Yates' comments are expected to contradict remarks from top Trump administration officials, who have described her conversation with McGahn in less serious terms.
President Trump took a shot at Yates on Twitter early Monday, suggesting she was involved in a leak of classified information.
On NBC’s "Meet the Press” on Sunday, U.S. Sen. Dianne Feinstein, the top Democrat on the Senate Judiciary Committee, hinted that Yates could make big news this afternoon:
"Well, Sally Yates is very much respected. She's a professional, she's not a politician, she has spent a lot of time in the department, she is very widely respected, and she apparently has some information as to who knew what when that she is willing to share. And that would be what she knew about Michael Flynn's connections to Russia and exactly what she knew they were."
Flynn was fired several weeks after Yates’ conversation with McGahn, but only after news of her warning leaked to the press.
The White House is planning to push back against Yates' testimony by framing her as a Democratic partisan who had it out for Trump, according to Axios.
Meanwhile, Trump on Sunday took to Twitter to discuss the media’s coverage of Russian meddling in the election:
Yates had initially been scheduled to testify before the House Intelligence Committee last month, but that hearing was cancelled by the panel’s chairman after a reported legal dispute between Yates’ lawyer and the White House about whether she was barred from testifying due to “executive privilege.”
Democratic operatives have floated Yates' name as a potential candidate for governor or other statewide office in 2018, but Yates has so far stayed silent on the issue.
The 56-year-old has deep roots in Georgia. She graduated from the University of Georgia School of Law and worked at the King & Spalding law firm in Atlanta before becoming an assistant U.S. attorney in Atlanta in 1989. Her father had served as a judge on the Georgia Court of Appeals.
She was promoted to U.S. Attorney for the district in 2010 before Obama tapped her for the No. 2 job at the Justice Department in 2014. She was confirmed by the U.S. Senate months later with overwhelming bipartisan support, including the backing of Republican Sens. Johnny Isakson and David Perdue.
Check back to myajc.com and Political Insider for the latest on Yates and her testimony.
Staff writer Greg Bluestein contributed to this report.