Lawrence Goodwyn, whose experience building cross-racial political coalitions in the 1960s led him to write an authoritative history of the rise of American populism in the 19th century, died Sunday at his home in Durham, N.C. He was 85.

The cause was emphysema, said his son, Wade.

Dr. Goodwyn’s 1976 book, “Democratic Promise: The Populist Moment in America,” became a standard text in colleges and something of a blueprint for activists hoping to harness grass-roots support to force political change. (An abridged version, “The Populist Moment: A Short History of the Agrarian Revolt in America,” was published in 1978.)...