This is the speech Biden has prepared for his entire life.

Over his near half-century in public life, Joseph R. Biden Jr. has made many speeches: good speeches, bad speeches, campaign kickoff speeches and concession speeches, speeches without proper attribution to original sources, speeches so impossibly Biden that no one could ever accuse him of lifting anything.

“No one ever doubts that I mean what I say,” Mr. Biden, 77, is fond of telling audiences. “The problem is, I sometimes say all that I mean.”

What he means to say on Thursday, as he accepts the Democratic presidential nomination in the most important — and likely most surreal — address of his career, has been something of a work in progress for several decades, since he charged onto the national stage as a 29-year-old senator-elect and sparked his first presidential speculation soon after.

And so, friends said, some elements of his preparation process were to be expected. There would be consultations with a coterie of family members and his longest-serving advisers, including his sister Valerie Biden Owens; his wife, Jill Biden; and his chief strategist, Mike Donilon. He would cycle through multiple drafts, reflecting a longstanding habit of tinkering until the end, often by hand.