By Daniel Schwen (Own work) [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons
Democrats have been trying to impeach President Trump for what they called quid pro quo for months. The problem is that they have no proof that the Ukraine president did what Trump wanted and Ukraine was given the foreign aid anyway.
Joe Biden, on the other hand, has admitted, on video, to engaging in actual quid pro quo with the Ukraine government.
The argument being made by Democrats is that Biden's corruption cannot be investigated or held against him because he is a presidential candidate.
Another thing worth noting is that President Trump was not impeached for bribery, quid pro quo, election fixing, or any other accusation thrown at him by the left. He was impeached under two very vague articles of impeachment of obstruction of Congress and abuse of power.