Ever since the disastrous debate at the end of June, President Biden was coming under huge pressure to drop out of November's presidential election. But the stubborn old man was resisting.
Senior Democrats pleaded with him in private, and when that had no effect, they leaked the conversations to the press to put public pressure on him. Thus we learned that a range of Democrats asked him to step down from Chuck Schumer to Nancy Pelosi and Barack Obama.
Then came the shocking assassination attempt on Trump on Sunday July 13th. The Trump campaign had been pushing a message that Trump was strong and Biden weak, and Trump stayed on message; despite the shock of the attack, and with his ear bleeding, with great presence of mind Trump raised his fist to give photographers an iconic "strong" image, before security bundled him into a car.
That seems to have sobered up Biden. The next day (July 14) the Biden campaign quietly suspended distribution and sales of mechandise. I believe that is when he decided to step down.
I don't think he told anyone apart from his wife and Kamala Harris. They remained silent during the Republican National Convention. They waited till it wrapped up on Friday July 19, left a day for decorum, and then Biden posted an announcement on twitter on Sunday July 21 that he was withdrawing from the election contest, together with an endorsement of Kamala Harris.
People like Pelosi and Obama put out warm statements, but no endorsement of Harris. Here is what Obama said:
We will be navigating uncharted waters in the days ahead. But I have extraordinary confidence that the leaders of our party will be able to create a process from which an outstanding nominee emerges.
"Creating a process from which a candidate emerges" sounds like a contest. When Obama was pressing Biden to step down, he likely had a candidate in mind, and it wasn't Harris. Ditto Pelosi.
But Pelosi and Obama got checkmated by Biden.
Biden was probably angered by the way Pelosi and Obama were pushing him to step down. And he wanted to choose his successor. If I am right that he made his decision to step down on July 14 and told Kamala Harris, that gave her a week to line up endorsements and fundraising.
Then the plan was rolled out with military precision. Biden released his statement and endorsement of Harris. Within an hour other endorsements and financial pledges came in. They staggered them over the next 48 hours to create a sense of momentum and give the press something to report. A social media campaign got launched. And they got lucky - ordinary Democratic supporters, giddy with relief, spontaneously came forward with small donations and posted home-made memes about her
Pelosi gave her endorsement by day three. Obama held out a week before endorsing Kamala too. By then, everyone else had, and she was already the nominee.