An adviser for the lobbying firm of former Gov. Haley Barbour could be a crucial witness in the President Donald Trump-Ukraine issue engulfing national politics.

Kurt Volker is a senior international adviser to the BGR Group, the lobbying firm founded by Barbour.

Ukraine hired Barbour and the BGR Group in 2017 to lobby on behalf of the country in Washington, D.C.

That same year, Volker began working as a special envoy to Ukraine — a volunteer position through the U.S. State Department similar to an ambassador.

Now, Volker is emerging as a key figure in the investigation into whether President Donald Trump pressured the country of Ukraine to dig up dirt on his political rival.

On Friday, Volker resigned from his position as special envoy to Ukraine. Volker's resignation came just hours after House Democrats announced plans to interview him.

Volker could have direct knowledge of events that are sure to dominate headlines for months to come — and it appears Volker has yet to speak publicly since the story broke.

Federal documents list Barbour as one five BGR lobbyists working on behalf of Ukraine from 2017 to early 2018. Volker was not listed as a lobbyist for Ukraine.

A spokesman for BGR, Jeffrey Birnbaum, said Volker never consulted for or advised BGR on Ukraine while serving as a special envoy to Ukraine.

"(Volker) is a consultant and adviser. He is not an employee," Birnbaum said.

The U.S. State Department has said Volker "has recused himself from all Ukraine-related matters in his other work,” according to NBC News.

Here's what we know

Democrats in the U.S. Congress have been taking steps toward impeachment after reports of a whistle-blower complaint surfaced.

A member of the U.S. intelligence community made the complaint, which was released earlier this week.

The complaint is primarily about a July 25 phone call between Trump and the newly elected president of Ukraine, in which Trump asked for a "favor."

According to a reconstruction of the phone call released by the White House, Trump asked the Ukrainian president to investigate former Vice President Joe Biden — the current Democratic front-runner in the 2020 race for President.

According to the whistle-blower complaint, Volker was in Ukraine a day after the phone call and "reportedly provided advice to the Ukrainian leadership about how to 'navigate' the demands that the President had made of (Ukrainian President Volodymyr) Zelenskyy."

Trump's lawyer calls out Volker on national TV

Earlier this year, Volker had been in communication with Trump's personal lawyer, former New York Mayor Rudy Giuliani, who was also meeting with Ukrainian officials, according to the whistleblower complaint.

The complaint claimed Volker was trying to "contain the damage" to U.S. national security caused by Giuliani acting as a back channel between Trump and Ukraine.

Giuliani went on Fox News Thursday night to dispute that — and also call out Volker by name.

“He should step forward and explain what he did,” Giuliani said.

According to Giuliani, the U.S. State Department — through Volker — was directing him to speak with Ukrainian officials.

"“Volker said, 'Would you meet with him? It would be helpful to us. We really want you to do it,'" Giuliani recalled.

According to his biography on BGR's website, Volker has "23 years of experience working on European policy under five presidential administrations" and was the Ambassador to NATO from 2008 to 2009.

The biography said Volker "previously served as Managing Director, International, for BGR," though it still lists him as a "senior international advisor."

According to federal disclosure documents, the government of Ukraine paid BGR $750,000 for lobbying in 2017 and early 2018.

Barbour was governor of Mississippi from 2004 to 2012 and was chairman of the Republican National Committee in the 1990s.

USA Today contributed to this report.

Contact Giacomo "Jack" Bologna at 601-961-7282 or gbologna@gannett.com. Follow him on Twitter @gbolognaCL.