Fruitcakes!

Loonies! Closet racists! True, it doesn't have quite the same ring as "Friends, Romans, countrymen", but that doesn't matter, because David Cameron has decided that if he wants to bury UKIP, rather than the other way around, he'd better do a bit of praising, rather than hurling insults at them.

What he actually said, when asked if he still thought they were fruitcakes etc, was: "Look, it's no good insulting a political party that people have chosen to vote for. We need to show respect for people who have taken the choice to support this party." Well, yes and no.

On the one hand, it makes sense for the Prime Minister to show respect for UKIP voters, since so many of them are people who voted for the Conservative Party in the past, but won't any more. It is true that UKIP is not merely a party of disillusioned Tories, but the polling suggests that Nigel Farage's outfit are picking up six times as many voters from them as they are from former Labour supporters or former non-voters.

What's more, a very significant number of the council seats lost in England last week would have been held or won if – obviously a completely hypothetical if – UKIP had not existed and those votes had been added to the Tory numbers.

But those (such as Lord Tebbit) who think the reason the Conservatives failed to secure a majority at the last election is that they were insufficiently right-wing are simply not supported by the numbers. There are issues, such as offering a referendum on the EU and being more bullish about immigration, which probably would have made the Tories more popular with some voters. There are also issues, such as gay marriage, maintaining foreign aid spending and the subsidy of alternative energy, which have alienated the Tories from some of their natural support.

Palaeo-conservatives are probably correct to say that Mr Cameron need not have lost this support. But they are wrong if they imagine that, on its own, it would have been enough to secure outright victory. Because, rightly or wrongly, the point of the Cameron leadership was to reposition the Conservatives to attract voters from the centre ground; the kind of people who voted for Tony Blair and who live in the marginals which decide general elections. The whole Tory modernisation (or, if you prefer, detoxification) strategy was to distance themselves from stereotypical traditionalists.

The point of UKIP, by contrast, is to embrace these people. This is, despite its invisibility in Scotland and many metropolitan areas of England, a big constituency; much bigger than media coverage would suggest. But it is not big enough on its own to win the Tories elections; though, as UKIP have demonstrated, it is big enough to lose them.

And while the election results were bad for the Tories for that reason, it should be noted that they were also fairly bad for the Labour Party and absolutely atrocious for the Liberal Democrats.

Yet it's particularly bad for the Conservatives because I don't think their current leadership can do much about this dilemma. Though it was criminally careless of them to go out of their way to alienate this body of opinion – and not, on many of the issues, necessary for modernising the party – they have done it now.

Bluntly, UKIP voters don't like or trust Mr Cameron, and even if he were to ignore the Lib-Dems and get an EU referendum set in stone before the next election (which he certainly should), it probably wouldn't be enough to win them round. In any case, Europe, though the cause where UKIP reflects public opinion best, is not the primary concern of many UKIP supporters, who tend to be more interested in immigration – a subject on which I happen to think they are flat wrong, but one which unfortunately the electorate does want to see a tougher stance on.

So without a change of leadership, the Tories cannot reunite the centre-right; they are, as a result, facing the problem that the SDP created for the Labour Party, when the centre-left vote was split. It's hard to see how they can attract enough voters to win elections outright without identifying, and bringing their way, some of UKIP's appeal, while at the same time holding on to those who think – as Mr Cameron used to (and, let's face it, still does) – that they are fruitcakes, loon- but we need not remind ourselves yet again of the derogatory terms.

I mean that last point seriously, because while UKIP probably does have a few supporters who are, well, you know what, so does every other party. And UKIP's policies are by no means outlandish. Some (such as EU withdrawal) I agree with, some (immigration and protectionism) I disagree with, some (like most of their economic policy) are utterly contradictory or, at the moment, non-existent. Other people will dislike most of their policies, or like the majority of what they see.

But what they aren't is particularly extremist. If this is Britain's version of the extreme right-wing, it's not exactly Golden Dawn, or even Le Pen's Front National, let alone a totalitarian, xenophobic movement like the Partito Nazionale Fascista or the Nazis – all of which could, given their authoritarian stance and avowedly socialist economics, as easily be described as left-wing extremists. UKIP is much more like the paramilitary wing of the golf club, or perhaps a sort of Provisional Rotary or Continuity Middle England.

It remains to be seen whether UKIP councillors – still a pretty meagre number, after all – do well in local government, or whether the party can overcome the inherent difficulties of the electoral arithmetic and muster a reasonable number of seats in Brussels and Westminster. At the moment, that is still unlikely.

The possibility for UKIP is that (as a sort of English National Party), they could exploit the public dissatisfaction with the main parties. At the moment, this rests entirely on Mr Farage who, despite his suspiciously foreign-sounding name, might have been designed by a committee to appeal to traditionalists in the Home Counties, and who is brilliantly playing the role of plain-speaking outsider.

But over time parties which are on the up tend, naturally, to attract able candidates, to develop more credible policies, to draw towards them the ambitious, the competent and, thereby, wider support. Who, after all, would have predicted the SNP's current standing 20 years ago?

At the moment, however, the real lesson of UKIP's rise is simply that they make it more or less impossible for the Conservatives to win an election.