Gordon Sondland, the U.S. Ambassador to the European Union, is set to testify on Thursday before Congress- despite White House attempts to block his appearance.

Democrats say Sondland served as an intermediary between President Donald Trump and Ukrainian president as the White House pressed Ukraine for “a favor.”

Trump wanted his Ukrainian counterpart to investigate possible misconduct by the son of former vice president, Joe Biden-one of Trump’s potential rivals in next year’s presidential election.

The Associated Press reports that Sondland will testify that he understood the White House was offering a quid pro quo.

On offer-a high-profile meeting with Trump in exchange for Ukraine’s announcing it would investigate Hunter Biden’s ties to a Ukrainian energy company.

Sondland will be testifying in response to a subpoena.

He initially declined after the U.S. State Department ordered him not to co-operate.

Trump and his party say hearings are politically biased.

Some Democrats say they may no longer seek the testimony of a whistleblower whose complaint started the impeachment inquiry.

There have been concerns over whether they can keep the whistleblower’s identity a secret.

Trump also asked the Chinese to investigate Hunter Biden’s work for a Chinese-backed investment firm.

China declined, saying it “will not interfere in the internal affairs of the U.S.”

Hunter Biden says he will resign from the firm’s board of directors.

He also pledged not to work for any foreign-owned companies if his father becomes president.

Trump and Republicans have repeatedly suggested that Hunter may have acted inappropriately without presenting any evidence.

David Charter on the impeachment inquiry of U.S. President Donald Trump

David Charter, US editor for The Times of London, talked with CGTN’s Roee Ruttenberg about the impeachment inquiry of U.S. President Donald Trump.