A forthcoming book that promises explosive new details on former President Joseph R. Biden Jr.’s mental and physical decline while in the White House has revived the subject of how his aides and top Democrats handled his decision to run for re-election.

The book, “Original Sin,” by Jake Tapper of CNN and Alex Thompson of Axios, chronicles how Mr. Biden’s advisers stomped out discussion of his age-related limitations, including internal concerns of aides, external worries of Democratic allies and scrutiny by journalists. Mr. Biden had long been gaffe-prone, but as he forgot familiar names and faces and showed his physical frailty, the authors write, aides wrapped him in a protective political cocoon.

At the same time, the book is so reliant on anonymous sourcing — very few aides or elected officials are quoted by name — that it reveals the enduring chill that Mr. Biden’s loyalists have cast over a Democratic Party still afraid to grapple publicly with what many say privately was his waning ability to campaign and serve in office. Already, Mr. Biden has begun pushing back against reporting on the end of his presidency, re-emerging for interviews to try to shape his legacy.

The book does not contain any explosive revelation that changes the broad perception of whether Mr. Biden, now 82, was fit to serve as president. Instead, it is a collection of smaller occurrences and observations reflecting his decline. The authors write about a “cover-up,” though their book shows a Biden inner circle that spends more time sticking its collective head in the sand about the president’s diminishing abilities than it does scheming to hide evidence of his shortcomings.

The New York Times obtained a copy of the book, which is set for release next Tuesday. Here are six takeaways.

Biden forgot names, even of people he had known for years.

During his 2020 campaign and throughout his presidency, Mr. Biden forgot the names of longtime aides and allies, according to the book.