WASHINGTON — Former U.S. special representative for Ukraine negotiations Kurt Volker has resigned as the head of the McCain Institute for International Leadership amid mounting fallout over his role in President Donald Trump’s push to get Ukraine to investigate his political opponents.

A former U.S. ambassador, Volker had already resigned as Trump’s Ukraine envoy two weeks ago, following a whistleblower complaint about Trump’s call with the Ukrainian leader prompted Democrats to begin impeachment proceedings. Volker's resignation from the institute comes just days after he testified before Congress for more than eight hours.

Volker’s resignation was announced Monday morning at a staff meeting, individuals familiar with the matter tell NBC News. In recent days Volker’s role in the Ukraine controversy had created concerns at the institute about whether he could continue to serve effectively as its executive director without being a distraction, those individuals said.

Volker confirmed his departure in a statement distributed by the McCain Institute, a Washington-based think tank named after Sen. John McCain, the Arizona Republican who died last year.

“I believe the recent media focus on my work as U.S. Special Representative for Ukraine Negotiations risks becoming a distraction from the accomplishments and continued growth of the Institute, and therefore I am stepping down,” Volker said.

Cindy McCain, the senator's wife, praised Volker in a statement, thanking him for “his dedication and hard work in helping to build the McCain Institute into the results-driven, action-oriented institution that it is today.”

A statement from Arizona State University, where the institute is based, said Volker would be on “paid administrative leave from his other university duties until further notice.”

Former National Counterterrorism Center Director Nicholas Rasmussen, who is also an NBC News national security analyst, will take over as acting executive director of the institute, the McCain Institute said.

The dual resignations mark a harsh fall for Volker, who served as ambassador to NATO in the Barack Obama and George W. Bush administrations. A long-serving diplomat and former White House official, Volker became executive director of the McCain Institute when it was founded by Arizona State University in 2012.

Trump’s personal attorney, Rudy Giuliani, and the State Department have both said that it was Volker who introduced Giuliani to a top aide to Ukrainian President Volodymyr Zelenskiy. Trump, in his call with Zelenskiy, urged him to work with Giuliani on opening an investigation into unfounded allegations of corruption by the Biden family and Ukrainian involvement in 2016 election meddling.

Until the whistleblower complaint brought new scrutiny on Trump’s interactions with the Ukrainians, Volker had mostly stayed under the radar since taking the envoy job in 2017. He worked for the Trump administration only part-time under special rules designed to enable private sector workers with particular expertise to serve in government temporarily.

The whistleblower complaint filed by a U.S. intelligence official in the wake of Trump’s call with Zelenskiy portrayed Volker and U.S. Ambassador to the European Union Gordon Sondland as trying to "contain the damage" to national security inflicted by Giuliani’s work in Ukraine.

But Giuliani has repeatedly said that everything he did on Ukraine was done at the direction of Volker and the State Department, and has cited text messages from the State Department thanking him for his “help.”

And text messages that Volker turned over to Congress last week painted a picture of Volker and Sondland, rather than trying to rein in Giuliani, working to facilitate his and Trump’s goal of getting the Ukrainian government to commit publicly to an investigation. The texts also show Volker and Sondland explicitly linking the inquiry to whether Ukraine’s president would be granted an official White House visit, even though Trump insists there was “no quid pro quo.”

In his opening statement to a closed-door deposition last week by Congress, obtained by NBC News, Volker said his “efforts were entirely focused on advancing U.S. foreign policy goals with respect to Ukraine.” He said that “at no time was I aware of or took part in an effort to urge Ukraine to investigate former Vice President Biden.”

But the texts messages appeared to tell a different story. They showed Volker and Sondland went as far as to draft and discuss specific language for what Zelenskiy should say, including a commitment to “initiate and complete a transparent and unbiased investigation of all available facts and episodes, including those involving Burisma and the 2016 U.S. elections.”

Burisma Holdings is a Ukrainian gas company. Former Vice President Joe Biden’s son Hunter joined its board in 2014, while his father was vice president and handling the Ukraine portfolio. Trump has made unsubstantiated allegations that the arrangement was corrupt.

There is no evidence of wrongdoing by either Joe or Hunter Biden. The president has also long promoted the baseless theory that Ukraine — not Russia — was responsible for 2016 election meddling.

It was unclear whether Volker would continue in another role as a “senior international adviser” at a Washington lobbying firm, BGR Group. A BGR group spokesman didn’t immediately respond to a request for comment.