Donald J. Trump’s presidential candidacy has stunned the Republican Party. But if he survives a late revolt by his rivals and other leaders to become the party’s standard-bearer in the general election, the electoral map now coming into view is positively forbidding. In recent head-to-head polls with one Democrat whom Mr. Trump may face in the fall, Hillary Clinton, he trails in every key state, including Florida and Ohio, despite her soaring unpopularity ratings with swing voters. In Democratic-leaning states across the Rust Belt, which Mr. Trump has vowed to return to the Republican column for the first time in...