“I need you and Mike at the house,” President Biden said late Saturday afternoon.

Mr. Biden was on the phone from his vacation home in Rehoboth, Del., with Steve Ricchetti, one of his closest advisers. The president was referring to Mike Donilon, his chief strategist. Soon, both men were in Rehoboth, socially distanced from the president, who was recovering from Covid.

From that afternoon and far into the night, the three worked on one of the most important and historic letters of Mr. Biden’s presidency — his decision to withdraw from his re-election campaign after top Democrats, donors, close allies and friends had pressured him relentlessly to get out.

He would not tell most of his staff until a minute before making his announcement to the world on Twitter on Sunday. Vice President Kamala Harris, whom Mr. Biden went on to endorse, also learned on Sunday, although it is unclear when.

Mr. Biden made his decision, a senior administration official familiar with his thinking said, in part because he had tried for weeks to flip the attention from his listless and at times incoherent debate performance last month back to his Republican opponent, Donald J. Trump.

But in the end, the official said, the president “couldn’t get there.”

On Saturday, Mr. Donilon focused on helping the president write a draft of the letter while Mr. Ricchetti focused on next steps: when to inform the staff, how to do it and who else would need to be notified.

The members of Mr. Biden’s family and his closest aides were notified on Saturday, while the letter was being written. It was finalized Sunday.

Vice President Kamala Harris, whom Mr. Biden went on to endorse, was notified on Sunday before the news was made public, the official said.

At 1:45 p.m. on Sunday — a minute before Mr. Biden posted his letter of withdrawal — the president told several of his advisers, including Anita Dunn, who manages communications strategy. He read the letter to them and thanked his staff for their service.

“Come to me with the work, and let’s get it done,” the president told them. The posting on X went out at 1:46 p.m.

On Sunday, some White House staff members were in shock. Others were crying. And others were relieved.

Announcing the end of his candidacy on X gave him the ability to do it “his way,” the official said, avoiding the intrigue and leaks that have plagued his campaign in recent weeks.