If Mr. Trump dispatches Ohio’s two-term governor and Florida’s young senator, it would suggest that he is close to wresting the Republican nomination. And that would quickly force other elected Republicans there to decide how tightly they want to embrace a candidate who, even with his victories in the primary, could prove repellent to the sort of up-for-grabs voters most coveted in these pre-eminent swing states. This calculation could also prove consequential in the Senate campaign, as both states have Republican-held seats at stake that could determine the balance of power in that chamber.

Even as Ohio’s economy has recovered since the Great Recession, manufacturing jobs have continued to vanish. Should Mr. Trump and Senator Bernie Sanders of Vermont on the Democratic side win Ohio, it will be in part because of their ferocious assault on the United States’ international trade agreements — a line of attack that also lifted them in Michigan. The economic conditions in the state have undermined support for free trade among voters and leaders in both parties. That augurs for a general election where the eventual party nominees will confront immense pressure to abandon the bipartisan free-trade consensus that has shaped every modern presidential administration.

Forget them not

Illinois and Missouri have a lot in common (besides being Midwestern states that share the Mississippi River). Both have been obscured by the action in Florida and Ohio, which are winner-take-all contests for Republicans and elimination races for Mr. Kasich and Mr. Rubio.

But Illinois and Missouri, which allocate a handful of delegates to the statewide winner and the rest to the top vote-getter in each of their congressional districts, could be just as important. If Mr. Trump overwhelms his competition in each, capturing every congressional district, he could effectively turn them into winner-take-all states.

But Mr. Kasich has aggressively campaigned in the Chicago area, and Mr. Cruz has stumped in nearly every corner of both the states. If together they can win a substantial number of the combined 26 congressional districts in both contests, it would limit Mr. Trump’s haul and mitigate the impact of his winning Florida, Ohio or both.