US President Donald Trump talks to reporters as he departs for travel to Florida from the South Lawn of the White House in Washington, DC, Oct. 3, 2019. Photo: Reuters / Leah Millis.

A top Pentagon official who oversees policy on Ukraine and Russia is expected to testify on Wednesday before the US House of Representatives panels leading an impeachment inquiry of Republican President Donald Trump.

The testimony of Laura Cooper, a career official, has been sought by lawmakers interested in her knowledge of alleged efforts by Trump to withhold hundreds of millions of dollars in aid to Ukraine until its president committed to investigating Joe Biden, the former vice president who is a top candidate for the 2020 Democratic presidential nomination.

Cooper, the deputy assistant secretary of defense for Russia, Ukraine, Eurasia, will appear voluntarily before the panels. The Pentagon has not blocked her from testifying.

Cooper’s testimony will follow that of William Taylor, the top US diplomat in Ukraine, who appeared before the impeachment inquiry on Tuesday.

Taylor testified that he was told by Gordon Sondland, the US envoy to the European Union, that Trump had linked the release of aid to public declarations by Ukrainian President Volodymyr Zelenskiy that he would investigate Biden, his son Hunter Biden’s tenure on the board of a Ukrainian energy company, and an already debunked theory that Ukraine meddled in the 2016 US presidential election.

In an opening statement to lawmakers that US media posted online, Taylor called the exchanges between Trump, his advisers and Ukraine a “rancorous story about whistleblowers … quid pro quos, corruption and interference in elections.”

In August, an unnamed whistleblower reported having received information from multiple officials that Trump, in a July 25 phone call, had asked Zelenskiy to commit to investigating the Bidens before he would release $391 million in security assistance approved by the US Congress to help combat Russia-backed separatists in eastern Ukraine.

Federal election law prohibits candidates from accepting foreign help in an election. Taylor’s testimony ran counter to Trump’s contention that there was no quid pro quo or wrongdoing. The president has accused Democrats of trying to oust him to prevent his re-election.

“There was no quid pro quo,” White House spokeswoman Stephanie Grisham said on Tuesday, as Taylor wrapped up his testimony. “Today was just more triple hearsay and selective leaks from the Democrats’ politically-motivated, closed door, secretive hearings,” she added.

Also on Wednesday, three Democratic US senators sent a “Freedom of Information Act” request to US Attorney General William Barr seeking any of his or Deputy Attorney General Jeremy Rosen’s correspondence referencing members of the Trump administration with Ukrainian, Turkish and other leaders.

They also asked for all records of interactions between Justice Department officials and the governments of Ukraine and China about Trump’s potential political opponents and any White House requests to investigate the Bidens.

In the letter, Senators Kamala Harris, Sheldon Whitehouse and Richard Blumenthal said the whistleblower complaint and recent testimony “raise serious concerns about the Justice Department’s involvement in politically-motivated investigations, at the behest of the White House.”