(COMBO) This combination of file pictures created on September 24, 2019 shows Democratic presidential hopeful former Vice President Joe Biden speaking during the third Democratic primary debate of the 2020 presidential campaign season at Texas Southern University in Houston, Texas on September 12, 2019 and US President Donald Trump waiting to speak during the 74th Session of the United Nations General Assembly at UN Headquarters in New York, September 24, 2019. - Growing numbers of Democrats are calling for the impeachment of US President Donald Trump over allegations that he sought political dirt on his potential 2020 presidential rival, Joe Biden.The political scandal stems from a July 25, 2019 telephone call between Trump and Ukraine's newly elected President Volodymyr Zelensky, a former comedian who ran for office as an anti-corruption crusader and reformer.Trump has acknowledged urging Zelensky during the call to investigate the business dealings in Ukraine of Hunter Biden, the son of Joe Biden, who is leading Trump in 2020 election polls. (Photos by Robyn BECK and TIMOTHY A. CLARY / AFP)