Mr Barr previously held the position under George H W Bush between 1991 and 1993 and looks set to lead the Justice Department once again should his confirmation hearings before the Senate Judiciary Committee conclude without a hitch.
A New Yorker and son to a Jewish father who had converted to Roman Catholicism, Mr Barr attended Columbia University where he completed a bachelor’s degree in government in 1971 and a master’s in government and Chinese studies in 1973.
After graduation he worked as an analyst and assistant legislative counsel for the CIA, studying law at night at George Washington University in the capital.
Completing his studies, he worked as a clerk for Judge Malcolm Wilkey on the US Court of Appeals for the District of Columbia Circuit between 1977 and 1978 and as an associate with Washington law firm Shaw, Pittman, Potts and Trowbridge.
He served on the Ronald Reagan administration’s domestic policy staff between May 1982 and September 1983 before returning to his old employer in private practice.
In 1989, he was appointed as assistant attorney-general of the US under the first President Bush, advising the Oval Office on the legality of executive decision-making, rising to the position of deputy before becoming acting attorney-general in 1991 when Dick Thornburgh resigned to campaign for a Senate seat in Pennsylvania.
Mr Barr developed a reputation as a controversial advocate of presidential power, backing US intervention in Manuel Noriega’s Panama and the FBI’s right to enter foreign soil without the consent of a host government.
He distinguished himself three days into his role as acting attorney-general when a group of 121 Cuban prisoners at a federal prison in Talladega, Alabama, took nine hostages in protest at their imminent deportation. Mr Barr sent in the FBI’s Hostage Rescue Team, who resolved the situation without a shot being fired.
Impressed, President Bush nominated William Barr for the job full time. Asked at his November 1991 confirmation hearing about Roe vs Wade, the Supreme Court’s landmark decision to legalise abortion, he suggested it should be overturned as it was a “legitimate issue for state legislators” while declaring he had no “fixed or settled views on abortion”.
Barack Obama’s future vice-president Joe Biden was chair of the Senate Judiciary Committee that day and praised Mr Barr for his candid answer, calling him ”a throwback to the days when we actually had attorneys general that would talk to you”, according to The Los Angeles Times.
As President Bush’s attorney-general, Mr Barr was known for his hardline stance on crime, reassigning 300 FBI agents to rein in gang violence.
“I believe deeply that the first duty of government is providing for the personal security of its citizens. Therefore I would naturally place the highest priority on strengthening law enforcement,” he said in 1992.
He had a hand in one of the most controversial uses of the presidential pardon in history, when Bush formally forgave six adminstration officials involved in the Iran-Contra affair, in which weapons were illegally sold to Tehran in defiance of an embargo, notably excusing the former secretary of defence, Caspar Weinberger.
Leaving the Justice Department with the arrival of the Bill Clinton administration, Mr Barr has worked in the corporate sector as executive vice president and general counsel to the GTE Corporation, leaving when the telecoms giant merged with Bell Atlantic to become Verizon in 2008. He has undertaken work with the Washington law firm Kirkland and Ellis since 2009.
President Trump saw an ally in William Barr when he told The New York Times in November 2017 there was more reason to investigate bribery accusations stemming from the sale of Canadian mining company Uranium One to Rosatom of Russia during Hillary Clinton's tenure as President Obama’s secretary of state than there was going after the Trump camp on collusion with the Kremlin.
He has since walked back these remarks before the Senate.
He also backed the president’s sacking of acting attorney-general Sally Yates in January 2017 over her refusal to enforce his “Muslim travel ban” and his removal of FBI director James Comey the following May as “the right call”.
Mr Trump’s decision to opt for Mr Barr rather than the current acting attorney-general Matthew Whitaker appears to stem from accusations that the president was seeking to install a Justice Department head hostile to special counsel Robert Mueller, Mr Whitaker having been outspoken in his criticism of the Russian election hacking investigation.
Despite describing Mr Mueller as “a good friend” saying he “would not be involved in a witch hunt” during his confirmation hearings, Mr Barr’s earlier criticism of the probe lingers in the memory and, as attorney-general, he would theoretically have the power to have Mr Mueller dismissed.
Perhaps of greater concern: William Barr is a keen bagpiper in his leisure time, even playing competitively on tour in Scotland as a youth.
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