How is Mr. Trump doing so well? He’s drawing on many moderate and secular voters who haven’t supported the anti-establishment but usually religious candidates who have fared well in Iowa. The same pattern emerges in national polls, which often show Mr. Trump faring best among self-described moderates.

The strength of a populist candidate like Mr. Trump, who opposes free trade and immigration, isn’t without precedent in New Hampshire. In 1992, Pat Buchanan, another anti-trade and anti-immigration candidate, won 38 percent of the vote against the incumbent president, George H.W. Bush. Four years later, Mr. Buchanan actually won the state, narrowly beating the eventual nominee, Bob Dole.

But the G.O.P. establishment then was not in anywhere near the danger it is now. This year, the “outsider” candidates, like Mr. Trump, Mr. Cruz and Ben Carson, possess as much organizational, financial and personal strength as the establishment candidates, or maybe more. This year’s schedule affords the party few opportunities to make a comeback: The contests after Iowa and New Hampshire — the Nevada caucuses, South Carolina and the predominantly Southern states on Super Tuesday — are all relatively favorable to conservatives. This year’s establishment candidates have shown far less strength, by any measure, than Mr. Dole or George H.W. Bush, who had the resources, name recognition and party backing to survive early setbacks.