Despite a barrage of criticism that he is an unelectable extremist, Mr Buchanan's impassioned message of social conservatism and economic protection had brought him on election eve to within striking distance of his goal, with polls putting him in a statistical dead heat with his closest rival, Bob Dole, and slightly ahead of the former Tennessee Governor Lamar Alexander. The millionaire publisher Steve Forbes is trailing in fourth place.
More important still, as a string of weekend rallies underlined, Mr Buchanan's supporters are committed - exactly the sort of voters who will brave what is expected to be a snowy election day to cast their ballots. His opponents' backers, by contrast, tend to be ruled by the head rather than the heart; the showings of Mr Alexander and Mr Forbes in particular could well be determined by the number of independents who bother to vote today.
The emotional momentum, though, is plainly with Mr Buchanan, who electrified 1,000 people packed into a Nashua convention hall on Sunday with some of the fieriest rhetoric of the campaign, as he preached his new "conservatism of the heart", and vowed to be a "voice for the voiceless" once installed in the Oval Office.
Amid chants of "Go Pat Go", the former commentator and speechwriter for Ronald Reagan lambasted the "bloodless corporate butchers" who were slashing their work forces even as profits rose, exporting American jobs to Mexico and other foreign parts. He savaged the United Nations, the North American Free Trade Agreement and other perceived threats to US nationhood, before turning his fire on the federal Government in Washington. Its bureaucrats, he said, "would be sent packing out of town, rolling across the Potomac bridges like old emigrants in covered wagons".
But what truly terrorises the party hierarchy is the onslaught Mr Buchanan unleashes against the hierarchy itself. The Republican leadership, he declared, had done nothing to help people who had lost their jobs: "What's the party doing? Just carrying water for the fat cats on K Street," he said, referring to the legendary abode of Washington lobbyists.
Splendidly as this sort of populism goes down in the "real America", it threatens to split the party asunder, setting the economic conservative wing which controls Congress against the social conservatives so influential during the primary season, and fraying ties with big business, a key Republican source of funds. A win here would send a triumphant Mr Buchanan to the next round of primaries in the South, where his message resonates especially loudly.
"I don't believe in class warfare," Mr Buchanan insisted yesterday. But 24 hours earlier that was precisely the impression he created as he spoke of a Washington establishment "in terminal panic" at his advance. "They're in their citadel pulling up the drawbridge. They've heard the shouts of the peasants from over the hill coming after them with pitchforks."
The contest here still seemed last night to be what has been dubbed the "Battle of the Three Shirts" - between Bob Dole the stuffed-shirt Senate grandee, Mr Alexander whose campaign badge has been a red and black plaid shirt, and Mr Buchanan, whose rhetoric has had some critics reaching to the brown shirts of Nuremburg for comparisons.