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Charles Dickens said that “life is made of ever so many partings welded together”.

When the great satirist died, his wishes to be buried in the place he loved most, Rochester, were ignored in favour of Westminster Abbey.

In two weeks' time, the people of Rochester look set to have a great parting with David Cameron’s Conservatives.

Commentators always overplay the significance of by-elections. This time though, they’re right. The Rochester by-election is unusually important, particularly for the Conservative party.

The Conservative party, a grand old institution, the roots of which date back to Robert Peel and the Tamworth declaration of 1834, is facing an existential crisis.

Peel created a modern political force capable of metamorphosing itself to survive in any political climate and prosper in the face of adversity.

Since 1834 the Conservative party has seen itself go through transition after transition, to accommodate itself to imperialism under the third Marquess of Salisbury, the post-war settlement and the rise of the welfare state under MacMillan and Heath, and then neo-liberal economics under Thatcher.

David Cameron’s attempts to modernise his party, through ideas like the big society and liberal reforms such as gay marriage, has alienated a large section of his political base.

One unintended consequence has been that many Tory activists, now including Members of Parliament, put their energy into UKIP.

Cameron seems incapable of dealing with the rise of UKIP. He is paralysed by indecision to the extent that he is unable to express what he really believes to be in the UK’s national interest.

Yesterday we saw his position laid bare at the dispatch box. He can’t even say whether he would vote ‘yes’ or ‘no’ in the referendum on Europe that he himself is proposing.

UKIP is a negative force. It’s nihilist. It has become the black cloud of negative energy that follows the Prime Minster wherever he goes.

People project onto it thoughts and feelings which are often mutually contradictory. This is why it is so hard for Cameron to characterise and engage constructively.

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It is why he has been forced back into the politics of the ‘nasty party’, with its recent focus on immigration and Europe, obsessions that sank John Major, William Hague, Iain Duncan Smith and Michael Howard.

It is for this reason that I believe that a win that gives UKIP their second MP threatens the very future of the Conservative Party for the first time in over a century and a half of electoral supremacy.

Yet Rochester is also a watershed election for Labour too. Recent polling shows that the NHS is the most important issue for the voters of the Medway - not Europe, not even immigration, not gay marriage nor the economy.

Despite this, the polls suggest UKIP is still ahead. They have managed to become the ‘all things to all people party’, projecting themselves as being the natural successor to the Conservatives and to a lesser extent, Labour.

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To quote Dryden, Nigel Farage is a “man so various that he seems to be, not one but all mankind’s epitome”.

If Labour, six months before a general election, cannot do well in a seat it held until 2010, where the main concern of voters is the NHS, it provides a very big challenge to Labour’s brilliant new general election supremo, Lucy Powell.

Ed Miliband was wise to ask her to re-energise Labour’s general election campaign but it's too late for her to change Douglas Alexander's campaign plan in Rochester. Her focus must be on the election next May.

Peel’s Tamworth declaration called on the Tories to stand against a “perpetual vortex of agitation.”

Rochester may not have buried Dickens, but in two weeks' time, after UKIP agitation, the voters there might well bury David Cameron.