President Donald Trump's July 25 phone call with the president of Ukraine is at the center of a whistleblower scandal that prompted House Speaker Nancy Pelosi to launch a formal impeachment investigation.

       According to a transcript of the call released by the White House, Trump repeatedly pushed for Ukrainian President Volodymyr Zelensky to investigate Joe Biden, a former vice president and potential 2020 political rival. He also asked Ukraine to investigate Biden's son, Hunter. The essence of Trump's argument is that Joe Biden was improperly trying to help Hunter, who served on the board of Ukrainian natural gas company Burisma, when Joe Biden pressured the Ukrainian government to fire the country's prosecutor general.

       There is no evidence of wrongdoing by either Biden.

       CNN confirmed that Trump ordered a hold on millions in military aid to Ukraine roughly one week before the call with Zelensky. Trump said he withheld the aid because he was waiting for European nations to contribute their fair share and that the delay wasn't related to the investigation request.

       Trump has insisted he did nothing wrong in his phone call with Zelensky, saying there was "no pressure whatsoever." He has called the impeachment inquiry "the greatest hoax," lashing out at Democrats, the media and the anonymous whistleblower who submitted a complaint to an intelligence inspector general.

       The inquiry's first public hearing began November 13, with testimony from diplomats Bill Taylor and George Kent. Marie Yovanovitch, the former US ambassador to Ukraine, testified two days later.