Jon Meacham, the presidential historian and biographer, has been helping to craft President-elect Joseph R. Biden Jr.’s speeches, according to multiple sources involved, including helping to write the acceptance speech that Mr. Biden delivered Saturday night from Wilmington, Del.

In that address, Mr. Biden spoke of a mission “to rebuild the soul of America, to rebuild the backbone of this nation, the middle class and to make America respected around the world again.” Mr. Meacham’s 2018 book, “The Soul of America: The Battle for Our Better Angels,” has long served as a touchstone for Mr. Biden, who read it and has reached out to Mr. Meacham in the past to discuss passages he liked.

Mr. Biden’s speech-writing process is run by Mike Donilon, the president-elect’s longtime adviser. But behind the scenes, Mr. Meacham has been playing a larger role than was previously known, both writing drafts of speeches and offering edits on many of Mr. Biden’s big addresses, including one he gave at Gettysburg last month and his acceptance speech at the Democratic National Convention in August.

TJ Ducklo, a spokesman for Mr. Biden, downplayed Mr. Meacham’s role. “President-elect Joe Biden wrote the speech he delivered to the American people on Saturday night, which laid out his vision for uniting and healing the nation,” Mr. Ducklo said. “Given the significance of the speech, he consulted a number of important, and diverse, voices as part of his writing process, as he often does.”