A very sexist and racist electorate, who basically voted Trump in because they were so upset by the election of Barack Hussein Obama
It remains to be seen if the racists and sexists will prevail.
Having said that, this might energize women who are on the fence and want to see the first female president. It might energize black voters. It will almost certainly energize Indian voters, possibly even all South-Asian voters. It will definitely energize voters who were worried about the age of the candidates. And now, suddenly, Trump has to go on the defensive about his age.
As long as all the democrats fall in line and push for Kamala, it might go really well. If Hillary Clinton goes out and works for Kamala, it could energize the people who are still angry about her loss, and can now channel that into the new option for a first female president. If Biden campaigns for her, it could reassure all the people who just wanted some stability.
OTOH, if there is infighting, and people trying to take her down so that they can become the nominee, then that could be trouble too.