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Before that moment, historians believe that no president had ever acknowledged Hispanics as a voting bloc, said Emilio Zamora, a historian at the University of Texas at Austin.

President William Howard Taft, who served from 1909 to 1913, may have met with a tiny group of Hispanic activists in El Paso, Texas, Mr. Zamora said. President Dwight D. Eisenhower likely shook hands with some Mexican-American voters in a campaign visit to South Texas in 1952. “But I think no president had ever publicly thanked Mexican-Americans in that manner,” said Mr. Zamora.

Fifteen hours after the historic meeting, Kennedy was dead.