By John Walcott and Sarah N. Lynch
WASHINGTON (Reuters) - The U.S. Justice Department's third-ranking official, Rachel Brand, plans to step down and take a senior job at Walmart Inc<WMT.N>, sources familiar with her decision said on Friday, at a time when President Donald Trump has taken aim at senior law enforcement officials.
Brand was next in line of succession to Deputy Attorney General Rod Rosenstein for oversight of Special Counsel Robert Mueller's investigation into potential collusion between Trump's 2016 presidential campaign and Russia and whether the Republican president has unlawfully sought to obstruct the ongoing probe.
Rosenstein oversees Mueller's investigation because Attorney General Jeff Sessions recused himself from the matter last year.
After just nine months on the job, Brand had grown increasingly uncomfortable with Trump's escalating attacks on the Justice Department and the FBI, which she and other law enforcement professionals feared was beginning to undermine the rule of law, according to sources familiar with her thinking.
The attacks have escalated in recent weeks as Republicans in Congress have criticized the handling by the Justice Department, FBI and the Federal Intelligence Surveillance Court of warrants for surveillance of a Trump campaign advisor, Carter Page, who had ties to Russia. Trump called the matter "a disgrace."
Another source, also speaking on condition of anonymity, said Brand will go to work as an executive at Walmart, the world's largest retailer.
A Justice Department spokesman declined comment.
Her resignation was first reported by the New York Times.
News of Brand's departure comes a week after Trump approved the release of a previously classified memo written by Republican lawmakers that portrayed the Russia investigation, initially handled by the FBI and now headed by Mueller, as a product of political bias against Trump at the FBI and Justice Department.
Trump also has criticized Sessions for recusing himself.
On Feb. 2, just hours before Trump approved the release of the Republican memo, Sessions offered praise for Rosenstein, the department's No. 2 official, and Brand, saying they "represent the kind of quality and leadership that we want in the department."
Trump fired then-FBI Director James Comey, who was leading the agency's Russia investigation, in May 2017, saying he took the action because of "this Russia thing."
The FBI's deputy director, Andrew McCabe, stepped down in January after Trump repeatedly criticized him on Twitter. McCabe's wife previously ran as a Democrat for a seat in Virginia's state Senate and received donations from then-Virginia Governor Terry McAuliffe, a close ally of Hillary Clinton and former president Bill Clinton.
The FBI has said McCabe was not involved at that point in the federal investigation into Clinton's use of a private email server while she was secretary of state. No charges were brought against Clinton.
A 1998 graduate of Harvard Law School, Brand worked in the White House Counsel's office under Republican former President George W. Bush, and helped draft his executive order creating the Department of Homeland Security.
(Reporting by Sarah N. Lynch and John Walcott; Additional reporting by Warren Strobel, Karen Freifeld and Anthony Lin; Writing by Will Dunham; Editing by Eric Walsh and Daniel Wallis)