This year, Democrats are dealing with an un-reelectable President whose only objective is to prevent Donald Trump from beating him in another election. One way to do this is to disqualify Trump from running by using "legal" machinations against him. If this is accomplished before the DNC Convention in August, Biden may announce that he is retiring from office and will not run again.
If Trump is not prevented from running (or from being on enough ballots to win the Electoral College), Biden may resign for health reasons and allow another nominee to be selected. That way, Biden could claim that Trump never beat him and the DNC could limit its downticket losses. If Biden refuses to leave office, the Democrats will be left with election interference/fraud as their only hope of staying in power.
If Trump is not prevented from running (or from being on enough ballots to win the Electoral College), Biden may resign for health reasons and allow another nominee to be selected. That way, Biden could claim that Trump never beat him and the DNC could limit its downticket losses. If Biden refuses to leave office, the Democrats will be left with election interference/fraud as their only hope of staying in power.