This one crime is more than enough to impeach a President. On 25 Jul 2019, Trump called Ukraine’s President and asked him 8 times to investigate Joe Biden’s son or else Trump would withhold the $250 million in aid desperately needed by Ukraine to fight off Russia. 

Mostly Verified but details needed: We know the phone call occurred. Giuliani admitted on CNN that he asked Ukraine to investigate Joe Biden’s son. Trump mostly confirmed these details too. Trump said that he spoke to Mr Zelensky about the problem of corruption and also about Biden and son Hunter among other issues.