Army Lieutenant Colonel Alexander Vindman, the White House National Security Council’s top Ukraine expert and a decorated Iraq war veteran, told impeachment investigators that President Donald Trump’s request that Ukraine investigate a political rival was an improper demand as he fended off Republican efforts to cast doubt on his competence and loyalty to the United States. “It is improper for the president of the United States to demand a foreign government investigate a U.S. citizen and political opponent,” Vindman said. Both Vindman and a second witness – Jennifer Williams, an aide to U.S. Vice President Mike Pence – said they were struck by the political nature of the requests made by Trump in a July 25 telephone call with Ukrainian President Volodymyr Zelenskiy that is at the heart of the impeachment investigation threatening Trump’s presidency. Vindman and Williams were among the U.S. officials who listened in during the call. Williams told the committee that Trump’s call with Zelenskiy was unusual and inappropriate because “it involved discussion of what appeared to be a domestic political matter.” Ahead of the July call, Trump had frozen $391 million in U.S. security aid approved by Congress to help Ukraine combat Russia-backed separatists in the eastern part of the country. Trump was seeking a Ukrainian investigation of former Vice President Joe Biden and his son Hunter, who had served on the board of a Ukrainian energy firm called Burisma. Later on Tuesday, former National Security Council Russia expert Tim Morrison and former diplomat Kurt Volker also testified. Volker, who was involved in efforts to get Zelenskiy’s government to commit to corruption investigations, said the United States should not have pursued allegations into the Bidens, and said the former vice president “is an honorable man.” Volker said he did not know that a request to investigate Burisma was effectively a request to investigate Biden. “In retrospect, I should have seen that connection different, and had I done so, I would have raised my own objections,” he said in prepared testimony.