Donald Trump is likely on the verge of losing the Republican primary, falling short of the number of delegates required to win the presidential nomination. But, as bullies are wont to do, Trump is now trying desperately to change the rules—to argue that the nomination should go not to the candidate who wins 1,237 delegates but to whoever comes closest. What’s wrong with that argument? Electing a U.S. president is not a schoolyard game, where goalposts change when bullies whine. There’s a reason a candidate has to make it to 1,237 votes to win the nomination. Each party’s goal is...