According to Mr Buchanan, Larry Pratt was "granted leave of absence" from the campaign after the charges were aired by the Center for Public Integrity, a watchdog group monitoring the candidates. He said Mr Pratt told him that although the accusations were "utterly false" he did not want them to be a distraction to the campaign.
Mr Pratt's links with the further fringes of the right are not in doubt. He is director of Gun Owners of America, a pro-firearms group which makes the better known National Rifle Association look weak-kneed. He has accused the government of "acting like a beast" in the 1993 siege at Waco. He met militia leaders after the FBI shot the wife and child of the white supremacist Randy Weaver in a siege of Mr Weaver's home in the Idaho mountains in 1992.
Whether or not Mr Pratt is guilty as charged, the affair underlines the vulnerability of a candidate who has largely enjoyed a free ride from the media, in part because Mr Buchanan, a former columnist and commentator, is one of their own, in part because he was never given a serious chance of winning.
That is changing, and there are drawerfuls of explosive Buchanan comments from the past, easily to be construed by opponents as extreme, racist and anti-Semitic, all available for instant resurrection by his Republican opponents now - and by the Democrats in the general election campaign, in the unlikely event that he captures the nomination.
In some recent New Hampshire polls, he has drawn almost level with Senator Robert Dole ahead of next Tuesday's key primary. Mr Dole is already running advertisements in the state suggesting Mr Buchanan is too extreme to be elected President.Reuse content