MOBILE, Ala. — It was immigration, not segregation, that brought some 20,000 southerners — far fewer than predicted — out for Donald Trump on Friday night, but the ghost of George Wallace loomed large. Wallace, an avowed segregationist, was the last presidential candidate to win electoral votes as a third-party candidate. The threat of Trump doing so, propelled by a hardline immigration stance that many have condemned as racist, looms over the Republican Party now as it did over the Democratic Party then, even as the enthusiasm of his following, for once, fell far short of expectations. Wallace carried five...