Kathryn Wheelbarger, one of the Pentagon's most prominent and respected policy officials, is resigning after three years in the job after President Donald Trump dropped plans to nominate her for an intelligence post, sources tell Reuters.
The Pentagon did not immediately respond to a request for comment.
Wheelbarger, who is highly regarded by national security experts in Trump's Republican Party and among Democrats, had been named by the White House on Feb. 13 to a senior intelligence position at the Department of Defense.
But in a surprise move last week, the White House instead announced plans to nominate Bradley Hansell, a former special assistant to Trump, to the position of deputy under secretary of defense for intelligence.
In her resignation letter, which was seen by Reuters, Wheelbarger said she trusted her colleagues would "continue to be guided by the U.S. Constitution and the principles of our founding, which ensure both our security and our freedom."
Both current and former U.S. officials accused the White House of passing over Wheelbarger because of her past work with the late Republican Senator John McCain, a fierce Trump critic and the former chairman of the Senate Armed Services Committee.
Wheelbarger handled the committee's intelligence portfolio and was well regarded by Congress, among both Republicans and Democrats.
"It looks like she failed the loyalty test somehow," said one former U.S. official, speaking on condition of anonymity.
A congressional source said it was disappointing that the Trump administration didn't recognize "one of its greatest assets."
"She was one of the few really thoughtful, compelling voices that people really seemed to respond to," the congressional source said, speaking on condition of anonymity.
Her departure will come as Trump attempts to overcome stiff Senate opposition among Democrats to his nomination of retired Army General Anthony Tata, a strong defender of Trump on Fox News, to the Pentagon's most senior policy position.
Tata has misportrayed former President Barack Obama as a Muslim and accused him of being a "terrorist leader" working to benefit Iran, according to now-deleted Tweets seen by Reuters.
Democrats, including Senator Jack Reed, the top Democrat on the chamber's Armed Services Committee, have signaled they would oppose Tata's nomination.
Senator Elizabeth Warren, who is also on the committee, called Tata "by far Trump's most unqualified & ill-suited senior defense nominee -- a high bar."
"An Islamophobic conspiracy theorist who called President Obama a 'terrorist leader' should not be #3 at the Pentagon," Warren said in a statement.