Few speeches have inflicted such lasting damage on the public good as the late Enoch Powell's utterance from 1968 when he predicted that post-war immigration into Britain would end in "rivers foaming with much blood".
It was why the words of Nigel Hastilow, the Conservative parliamentary candidate for a Midlands seat not far from Powell's old Wolverhampton constituency, in which he insisted Powell had been right, touched off such alarm.
By resigning, Mr Hastilow has saved his party leader from an exquisite dilemma, which is what to do with a candidate who blunders into a topic that his leader has raised himself only recently.
But as the furore has shown, the Conservatives are not yet into calm waters on immigration, a subject that has powers to inflame public passions but which at the same time has proven unpredictable as a source of votes. As various recent surveys of public opinion have shown, people remain concerned about immigration but, at the same time, wary of those who seek to exploit it as an issue.
Mr Cameron's predecessor, Michael Howard, failed to understand this distinction when he put immigration at the heart of the Conservative election campaign in 2005.
Mr Cameron has learnt important lessons from that debacle, and on the whole has been distrustful of immigration as a Conservative cause, viewing it, probably rightly, as a debased currency electorally.
The problem is that if he avoids the subject altogether, he risks being accused of political cowardice, whereas if he opens it up, he risks resurrecting the ghost of Powell, just as Mr Hastilow appeared to seek to do.
As Mr Hastilow has fallen on his sword, the Tory leader has been spared a confrontation with Mr Hastilow, and by extension, his own right wing, keen to tar Mr Cameron as a metropolitan liberal with an overdeveloped sense of political correctness. Mr Hastilow's withdrawal has also deprived Labour of an opportunity to attack the Tories as soft on racists.
But lessons need to be drawn – and spelled out in public. Lest others seek to tread in Mr Hastilow's footsteps, the Tory leader needs again to make it clear that while he is not squeamish on the subject of immigration, he sees a clear distinction between a responsible debate on a subject of national interest and a return to the kind of poisonous and inflammable language that Powell injected into the issue.
Should he fail to do so, the Conservatives will be back where they were in the public's estimation – condemned as a party that cannot be trusted not to exploit people's basest and most foolish fears over immigration, and in the most opportunistic manner.