Some pundits and a whole lot of excited Democrats project the once fiery-red Lone Star State to be on the verge of going blue — or at least turning a shade of purple. Such a flip would upend national politics and threaten to put Republicans out of reach of the White House for a generation; Texas’s 38 electoral votes put it just behind California, which has 55, for the most. But the battle for Texas is likely to be a prolonged affair, a multiyear war of attrition that both parties are gearing up for. Texas has long been on the list of Democratic holy grails, usually on the premise that Hispanic immigration and in-migration from blue states will make a Democratic shift inevitable. In recent years, however, high-profile elections in the state have been close...