WASHINGTON – Defense Department official Laura Cooper is scheduled to testify Wednesday in the House impeachment inquiry about the U.S. withholding military aid from Ukraine while President Donald Trump sought the investigation of his political rival.

Cooper, who serves as the deputy assistant secretary of defense for Russia, Ukraine, and Eurasia, was set to appear behind closed doors before the House Intelligence, Foreign Affairs, and Oversight and Reform committees. Her testimony had been scheduled for Friday, but was moved to Wednesday.

Congressional investigators want to ask Cooper, who oversees Ukraine policy, about why the administration withheld military aid at a time when Trump was urging Ukraine President Volodymyr Zelensky to investigate former Vice President Joe Biden and his son, Hunter.

Bill Taylor, the top U.S. diplomat in Ukraine, told lawmakers Tuesday about the potential linkage between the aid and investigations. 

Rep. Stephen Lynch, D-Mass., told reporters that Taylor's testimony was a "sea change" that could "accelerate" the impeachment process.

Cooper's appearance isn't certain. Last week, Defense Secretary Mark Esper notified congressional chairmen that he would not cooperate with a subpoena for documents from the Defense Department. The White House has vowed not to cooperate with the impeachment inquiry, and has called it a "partisan" effort and a "coup."

But the State Department also directed Taylor not to testify, and he appeared under subpoena.

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