The interviews could yield more fodder for Democrats’ impeachment drive over a whistleblower’s allegations that Trump leveraged $400 million in aid to secure a promise from Ukrainian President Volodymyr Zelenskiy to investigate political rival Joe Biden, and his son Hunter, who sat on the board of a Ukrainian energy company.

Trump has denied wrongdoing and assailed the probe.

Among those due to testify: Gordon Sondland, the U.S. ambassador to the European Union who was involved in efforts to get Ukraine to open the investigations, and Masha Yovanovitch, who was abruptly recalled from her post as U.S. ambassador to Ukraine in May after Trump supporters questioned her loyalty to the president.

Trump’s White House could formally tell House Speaker Nancy Pelosi as early as Monday that it will ignore lawmakers’ demands for documents until the Democratic-controlled House of Representatives holds a vote to formally approve the impeachment inquiry.

Pelosi says a vote is not needed, but Democrats say she would prevail if one were held, although very few Republicans would be expected to side with the Democratic majority. Congress returns to Washington on Oct. 15 after a two-week recess.

“There’s no question in my mind that she would have the votes,” Democratic Representative Jim Himes said on CBS’ “Face the Nation” on Sunday.

The impeachment investigation could lead to the approval by the House of formal charges against Trump.

A trial on whether to remove him from office would then be held in the Republican-controlled Senate, but few Republicans have broken ranks with Trump so far for asking Ukraine and China to launch investigations of Biden, a former vice president and leading contender for the Democratic nomination to face Trump in the November 2020 presidential election.