Donald Trump, fresh off his Northeast sweep, declared himself the “presumptive nominee.” He presumes too much.

For all the news of Mr. Trump’s victories and Ted Cruz’s veepmate, the essential action of this GOP contest continues to take place far from the media lights. It’s happening in towns like Harrisonburg, Va., where Republican voters will gather this weekend to pick a slate of delegates for the Cleveland convention. That’s the action that matters, and it’s not going Mr. Trump’s way.

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