With the prospect of a contested convention looming over the Republican presidential primary, Donald J. Trump on Sunday complained about the party’s rules requiring a candidate to have a majority of delegates to clinch the nomination outright. Asked by George Stephanopoulos on ABC’s “This Week” why he should be “guaranteed the nomination” if he failed to amass the 1,237 delegates needed to win it on the first ballot, Mr. Trump said that he might be unable to clear that threshold. But he blamed the number of contenders in the Republican field. “If I’m a little bit short – and one...