Certain politicians try to dress up their rhetoric about immigration as a concern for national security. But they’re really peddling an old, racist theory that has already fueled several massacres in the United States, an alliance of eight anti-hate groups report. They say a slew of public statements and some legislative and state actions are based on the “great replacement” theory — a belief that shadowy, often-Jewish actors are orchestrating mass immigration by people of color to break the dominance of white people in American society. The theory has been connected to racist massacres in El …