Tim Phillips has resigned as the president of Americans for Prosperity after the Koch-backed conservative organization made clear he was no longer welcome following an internal investigation into unspecified personal impropriety.

Americans for Prosperity is the well-funded grassroots and voter turnout arm of the Koch network of political and policy groups, and Phillips was its public face for nearly 17 years before resigning Wednesday. Phillips had been on paid leave since just before Thanksgiving, when the group became aware of a personal indiscretion. After investigating the matter, Americans for Prosperity concluded Phillips’s “behavior” was “incompatible with our organization’s values,” and he was allowed to resign.

“While the underlying issues involved are personal in nature, Tim’s actions violate our core principles and make it impossible for him to continue in this role,” Mark Holden, board chairman of Americans for Prosperity, said in a statement shared first with the Washington Examiner. “This has caught us all by surprise. But we believe Tim’s departure is necessary for him, his loved ones, and for AFP.”

According to sources familiar with Americans for Prosperity, the weeklong, in-house investigation into Phillips’s actions determined that “this was a personal issue and did not to our knowledge impact anyone internally at AFP.” Americans for Prosperity officials are declining to detail the exact nature of the conduct that led the group to seek Phillips’s resignation, but sources say “it was a matter of integrity that violated” the group’s “core principles.”

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Phillips, 57, did not respond to a request for comment Wednesday afternoon but issued a statement announcing he had left the organization. "This morning, I announced my resignation as president of Americans for Prosperity in order to focus on some challenging personal matters that require my full attention,” he said. “It is difficult to leave this organization, but doing so now is in everyone's best interest.”

Emily Seidel, Americans for Prosperity's CEO, was to immediately assume Phillips’s responsibilities. Sources say the transition should be seamless because all financial and strategic day-to-day decision-making and operations have been the responsibility of the group’s CEO or chief operating officer for the past 10 years. However, Phillips’s departure is still expected to rock the insular world of prominent conservative political organizations headquartered in Washington.

That is especially so in this case because Americans for Prosperity operates under the high-profile Koch umbrella. The founding of Americans for Prosperity and a host of other political and policy groups was financed by Kansas billionaire industrialists Charles and David Koch. David Koch died in 2019 and had stepped away from involvement in the groups due to failing health prior to his death.

For more than a decade and a half, a lifetime by Beltway standards, Phillips was the face of Americans for Prosperity.

He served as its ambassador at major conservative conferences, consulted with conservative activists and wealthy donors, and interacted with the media. In election cycles that saw the Koch groups active and investing up to hundreds of millions of dollars, mostly on behalf of Republicans, Phillips often led the public charge as the representative of the network group, which has affiliates spread across key battlegrounds and specializes in grassroots mobilization and voter turnout.

Americans for Prosperity is planning for another cycle of aggressive politicking in next year’s midterm elections, with Republicans poised to win majorities in the House and Senate.

In 2020, the group, along with affiliated super PAC Americans for Prosperity Action, played in nearly 275 races and made more than 59.1 million voter contacts. Of the contests Americans for Prosperity and its super PAC engaged in, 78% were won by candidates the organization supported. This past November in Virginia, it reached more than 100,000 voters via direct-mail and person-to-person contacts, flying in conservative activists from around the country to campaign for the GOP slate.

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The Koch network did not always see eye-to-eye with former President Donald Trump. But Americans for Prosperity was out front supporting Trump’s three Supreme Court nominees and brags of supporting the confirmations of Justices Amy Coney Barrett, Neil Gorsuch, and Brett Kavanaugh.

"AFP has never been stronger," Holden said.