Ed O’Callaghan, a Clifford Chance partner in New York who was in the running to replace ousted U.S. Attorney Preet Bharara and who previously served as a senior adviser to U.S. Sen. John McCain’s 2008 presidential campaign, has left the firm to join the U.S. Department of Justice.

O’Callaghan, who also assisted on President Donald Trump’s transition team, according to the New York Post, started his new role—as principal deputy assistant attorney general—on Monday in the DOJ’s National Security Division. He will be responsible for helping to shape U.S. national security and enforcement priorities.

He spent six years at Clifford Chance, having joined in 2011 as a partner in the firm’s government investigations and white-collar criminal defense practice. Previously, O’Callaghan worked for Nixon Peabody, where he headed up the firm’s government investigations and white-collar defense group.

In 2008, while working on the McCain for President campaign, O’Callaghan acted as the campaign’s spokesman regarding Alaska state investigations involving vice presidential candidate Sarah Palin.

Before that, O’Callaghan worked for the U.S. Attorney’s Office for the Southern District of New York, where he was co-chief of the terrorism and national security unit.

During his time with Clifford Chance, O’Callaghan represented senior executives and officials of JPMorgan in connection with $6.2 billion (£4.7bn) in trading losses in 2012; O’Callaghan acted for Achilles Macris, who was head of the London branch of JPMorgan’s chief investment office, where the trader nicknamed the “London Whale,” Bruno Iksil, worked.

He also acted for former top FIFA official Jeffrey Webb, who was arrested for corruption charges in 2015. Webb, who pleaded guilty later that year, was among several officials arrested in 2015 on corruption charges following an inquiry by the Federal Bureau of Investigation.

In addition, he helped secure a major win in Washington, D.C., for Dutch aerospace firm Fokker Services in 2014, in a case relating to alleged U.S. sanctions violations.

“Although he will be missed by our firm, this is a great opportunity for Ed as he returns to government service,” said a spokesperson for Clifford Chance. “We want to thank Ed for his many substantive contributions to our firm, notably in connection with high-profile global investigations.”