DURHAM, New Hampshire — First Donald Trump’s Republican critics banked on one of his opponents catching a spark in Iowa. Then they said New Hampshire was where they’d make their mark. Eventually, advisers to Trump’s rivals maintained, someone would emerge from the field of lower-polling candidates to get a clear shot at Trump. After the past week, that idea has never looked more out of reach. In Iowa, new polling shows Trump not only maintaining his enormous lead in the first caucus state, but expanding on it — stretching his support to 51 percent of likely Republican caucus-goers. And in...