Brett McGurk, the Obama administration’s pick to be the ambassador to Iraq, withdrew his nomination on Monday in the face of mounting opposition in the Senate.

Senate Republicans last week expressed doubts about McGurk after a racy e-mail exchange surfaced between McGurk and a Wall Street Journal reporter covering him. The e-mails between McGurk and reporter Gina Chon — whom he later married — date from when McGurk was working in Iraq for the National Security Council under President George W. Bush and Chon was stationed in Baghdad.

In a letter to the White House last week, six Republican members of the Senate Committee on Foreign Relations wrote that such “unprofessional conduct ... will affect the nominee’s credibility in the country where he has been nominated to serve.”

The White House expressed regret over McGurk’s decision. “We greatly appreciate Brett’s years of service on behalf of the United States, to include tireless and effective leadership in Iraq from the height of the war to the moment our last troops left Iraq in December and through the challenging transition earlier this year,” said National Security Council spokesman Tommy Vietor. “While we regret to see Brett withdraw his candidacy, there is no doubt that he will be called on again to serve the country.”

In a letter announcing his decision, McGurk said the controversy over the e-mails represented a significant part of his reasons for withdrawing. His wife resigned from the Wall Street Journal last week after acknowledging that she had shown McGurk unpublished articles, a violation of the newspaper’s policy.

“The most difficult part of this process, however, was watching my wife become a part of it,” he wrote, according to a copy of the letter obtained by the Associated Press. “She is the most precious thing in the world to me, and the depiction of our relationship has been both surreal and devastating.”

The e-mails were not the only hurdle to Senate confirmation for McGurk. Sen. John McCain (R-Ariz.) had previously criticized his handling of U.S. policy in Iraq, including the inability of U.S. and Iraqi negotiators to reach a deal that would have left a small U.S. military presence behind. All U.S. combat troops left Iraq last year after those negotiations broke down.