“This president and his — the Ku Klux Klans and the rest of them, they think they’ve beaten us again. But they have no idea — we’re just coming back,” Mr. Biden told the congregation.

“I thought you could defeat hate,” he said. “But hate only hides. It never fully goes away,” he added, pointing to the outbreak of white supremacist violence in Charlottesville, Va., in 2017. “And they asked the president what he thought, and he said, ‘There were very fine people on both sides.’”

Referring to what he called a historic inflection point — when the police aimed fire hoses at civil rights activists in Birmingham, Ala., during the 1960s, galvanizing more of the country behind their movement — Mr. Biden said that the country had reached a similar moment.