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WATCH: Alexander Vindman says Trump-Zelensky call was ‘improper’

A White House aide tells lawmakers that what he heard on a July phone call between President Donald Trump and the new Ukrainian president was “improper.”

Alexander Vindman testified before House lawmakers on Nov. 19 as part of the impeachment inquiry into President Donald Trump. Watch in the player above.

Lt. Col. Alexander Vindman is testifying Tuesday in a public hearing in the House impeachment inquiry into Trump’s pressure on Ukraine to investigate his Democratic political rivals as he withheld aid to the East European nation.

Vindman is a U.S. Army officer detailed to the National Security Council. He listened in on the July 25 call at the center of the impeachment inquiry. Trump asked the new Ukrainian president to look into whether Ukraine interfered in the 2016 election and wanted the country to investigate Joe Biden and his son Hunter.

MORE: A guide to the impeachment hearings

Vindman said it was “improper” for Trump to demand a foreign government investigative a U.S. citizen and political opponent.

Vindman is one of several witnesses coming before the committee this week. He and the other witnesses have already testified behind closed doors.

Trump has denied doing anything wrong.