Voters have gone to the polls in droves during the Republican primaries, putting turnout on a record pace, yet the decision about who will be the ultimate nominee is increasingly likely to rest with fewer than 2,500 party insiders.

That has put GOP leaders in a historic and potentially explosive bind: Explaining that while Donald Trump has the most votes—some 1.9 million more than his closest rival—the rules could allow those officials to nominate someone else.

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