In Chapter 11 (page 286 of my edition of the book), Zinn quotes the famous historian C. Vann Woodward as saying that the two races were never closer than they were "during the Populist struggles." Zinn does not dispute this statement.
However, Zinn does point out that there was a great deal of racial tension even among the Populists. The Populists were ambivalent on the issue of race. For example, he points out that a Populist-controlled legislature in Georgia passed more anti-black bills than any other legislature had in a single year. Yet, in 1896, the Populists in Georgia had a platform that denounced lynching. Some Populist groups had large black memberships while others did not.
The record, then, is mixed. Even so, Zinn seems to agree with Woodward that the two races came closer together during the Populist era than they ever had before.
The Populist party, also called the People's Party, tried to unite African-Americans and whites but was not successful in doing so. Part of the problem was that African-Americans were not given leadership roles in the movement. For example, the Colored Farmers National Alliance was led by whites, not African-Americans. The races were also divided politically in the south. African-Americans were reluctant to vote for Democrats, as they saw Democrats as the party of segregation and were generally loyal to the Republican Party. In addition, whites and African-Americans were in different positions in the south, as whites were generally farmers, while African-Americans were sharecroppers.
However, there were some successes, as the Texas People's Party was founded as interracial in 1891. The historian C. Vann Woodward states that "Never before or since have the two races in the South come so close together as they did during the Populist struggles." However, as the Populist party began to promise some degree of racial solidarity, the Democrats played on whites' racial fears to win them back to the Democratic party. Whites, struggling themselves with the crop-lien system, had common cause with African-Americans, who were also struggling. However, whites were often manipulated by politicians to intensify their feelings of racism and to prevent the races from uniting, and southern governments made it increasingly harder for African-Americans to vote. Segregation in the south intensified, and the races did not act in unison in the populist struggles.