The Southern Poverty Law Center said Thursday that it had fired its co-founder and chief trial lawyer, Morris Dees, after nearly a half-century, during which he helped build the organization into a fearsome powerhouse that focused on hate crimes and with an endowment that approached half a billion dollars. The group’s president, Richard Cohen, did not give a specific reason for the dismissal of Mr. Dees, 82, on Wednesday. But Mr. Cohen said in a statement that as a civil-rights group, the S.P.L.C. was “committed to ensuring that the conduct of our staff reflects the mission of the organization and...