The Democratic president had won a strong victory in the November election, and now the nation’s best-known civil-rights leaders were urging him to take strong federal action to override a number of states where conservative lawmakers and sheriffs were impeding Black and brown Americans from voting. The president agreed with them in private — but he also insisted that he had to go slow, because a voting rights showdown might cost him centrist votes he needed for his bold economic agenda to fight poverty, expand health care and overhaul immigration. But within weeks, something dramatic happened…