Trump Adds Conservative Operative David Bossie as Deputy Campaign Manager

— -- Donald Trump has hired veteran conservative operative David Bossie to serve as deputy campaign manager, two senior-level campaign sources confirm to ABC News.

Bossie has investigated the Clintons for several years.

He was the president of conservative political group Citizens United, but he has reportedly taken a leave of absence from the group for the duration of the campaign.

A series of State Department emails released last month which shed light on the contact between the Clinton Foundation and Hillary Clinton's inner circle while she was secretary of state was released as part of a public records lawsuit by Citizens United.

The Washington Post first reported about Bossie's new role in the Trump campaign.

In a statement released Thursday night, the Clinton campaign said, "David Bossie is so craven and maniacal that in the heyday of the overreaching, Gingrich-era Congress, the top Whitewater conspiracy theorist in the House had to fire him for doctoring evidence. He has devoted his career ever since to trying to tear down Hillary Clinton. For months now, Citizens United has been acting as an arm of the Trump campaign, and this hiring of Bossie now makes it official. This is just the latest sign that Donald Trump has put the most extreme elements of the right-wing fringe in the driver's seat of his campaign."

ABC News' Liz Kreutz contributed to this report.