The Tories are clearly in a state of panic, if not sheer funk, over immigration.
Vince Cable's attack, invoking Enoch Powell, was the most visceral he has yet made on his Tory colleagues in the Coalition.
To be fair to David Cameron, he instinctively belongs to the decent Tory tradition of Ted Heath and Iain Macleod, both of whom ostracised Enoch Powell after his notorious and wicked speech in Birmingham in April 1968 when, discussing immigration, he warned: "Like the Roman, I seem to see the Tiber foaming with much blood".
As Ted Heath noted later, the heinous implication of Mr Powell's speech (which was carefully prepared - it was no outburst) was the mass expulsion of all black people from the UK.
That terrible suggestion was in part responsible for the Tories being continually demonised as the "nasty party." The issue had earlier emerged in 1961, when the Tory government led by Harold Macmillan controversially restricted Commonwealth immigration to Britain through a voucher system; a Commonwealth country passport was no longer enough to enter the country.
Mr Powell's speech would have been even worse had the Tories been in government, but they were in opposition at the time. Even so, Mr Powell instantly became an unlikely national figure.
The most cerebral of politicians - he was a professor of Greek in his mid-twenties - he had been obscure and unsung. Now, for many, he was an instant hero, and for a few weeks he came close to destroying his party. He had dared to articulate the fears of many ordinary voters who thought politicians were out of touch. Dockers and other workers marched and rallied in his support.
But for the majority in his party he simply became a leper overnight. He was seen as totally unfit for public office. He was forced out of mainstream UK politics; he tried to revive his shattered career in Ulster, without any significant success.
Ever since, the Tories have been wary of the immigration issue. This is what makes Mr Cable's intervention so devastating.
Nobody could claim that today's Tories want the mass expulsion of immigrants. They have managed to get themselves mired in the current mess simply because they are running scared of Ukip. I reckon that Mr Cable is well aware of this, but unlike his leader Nick Clegg he cannot contemplate a second Coalition with the Tories. He is using the immigration issue to make certain that there cannot be a second Tory-LibDem agreement.
Various Tories are actually helping him make the divide between the two coalition partners ever wider.
One Tory MP, Philip Davies, said yesterday that he had "nothing but contempt" for Mr Cable "and his typical LibDem duplicity".
While it was cynical of Mr Cable to evoke that disgraceful speech of 45 years ago, he has clearly succeeded in opening up a festering wound. Ukip's skilful, if pernicious, manipulation of the immigration issue is deeply alarming for the Tories.
Outbursts such as the recent one by the junior minister Anna Soubry, insulting the Ukip leader Nigel Farage, indicate how rattled the Tories have become.
Ukip have been raising populist fears about waves of Bulgarians and Romanians arriving in the UK next year, when controls are eased by the EU. In response to this Ukip campaign, the Coalition is to introduce a three month freeze on benefits for non-working immigrants. Nick Clegg has endorsed this policy.
That is why Mr Cable is so obviously determined to prevent his leader doing another deal with the Tories in 2015.
Here, incidentally, is where we can currently find the clearest of dividing lines between the UK Government in Westminster and the Scottish Government in Holyrood.
The Scottish Government understands significant reductions in net immigration can have very negative effects on an economy, as yesterday's timeous report by the National Institute of Economic and Social Research showed.
How ironic it will be if immigration policy turns out to be a major issue in the final months of the referendum campaign.
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