When big parties lose by-elections, or scrape home with majorities sliced to the bone, old excuses are trotted out.

Voters have the blues; they're in the mood for a protest gesture; novelty has seduced them. But never mind: all will be well when a general election presents a real test of loyalties.

South of the Border, the grains of truth in such claims are being swept away by the Ukip tide. Nigel Farage and his party have been around for a couple of decades. Incoherent and unpleasant their policies might be, but they can no longer be dismissed as passing fads. The main Westminster parties are held in something like contempt; the obsession with immigration strikes a chord; this protest goes deep; and nothing important is liable to change before next May.

Tories no longer pretend those tempted by Ukip are in any mood to return to the fold. David Cameron has promised them his in-out referendum on the EU. His ministers have done their tough talking over immigration. Just days ago, the Prime Minister tried the oldest trick in the Conservative book with promises of tax cuts. Nothing has worked.

The Tories are reduced now to elaborate (if lame) scare stories. It might sound plausible to claim a vote for Ukip only damages Mr Cameron and allows Ed Miliband the chance to occupy No 10. That stratagem falls apart, however, if you then observe - accurately enough - that Labour did shockingly badly in Heywood and Middleton. Ukip is not, if it ever was, a private argument among past and present Conservatives.

If Mr Miliband didn't know as much before, he knows it now. The Greater Manchester seat was supposed to be an approximation of a northern redoubt for Labour. The party was defending a majority of close to 6,000, despite having lost a lot of support in 2010. Now Liz McInnes emerges just 617 votes ahead of Ukip. Had it not been for former LibDem supporters switching - or so we must presume - Mr Farage's smugness would be worse than ever.

Douglas Carswell's return in Clacton can be explained away, just about. You could simply say a popular local MP - it takes all sorts - has had his mandate confirmed.

He remains the same eurosceptic he was in 2010. He's still keen to privatise the NHS, even if he avoided campaigning on that theme. In every sense save party colours, you might argue, he remains a quintessential south-coast Tory of the sort often labelled interesting.

But Mr Carswell has emerged from his by-election with a slightly increased majority despite a vastly reduced turn-out. He has trounced his old party, cut Labour's support from 10,799 to 3,957, and reduced his LibDem opponent to the status of a novelty candidate with just 483 ballots. Above all, Ukip's first MP has claimed fully 59.7 per cent of the vote. His was more, much more, than a personal triumph.

Three facts are worth recording. First, once you set aside the excuse these were just by-elections, you see there are no longer Tory or Labour heartlands in England. Mr Farage is right about that. Ukip is drawing support from across the familiar Westminster divide. Some very big majorities will endure in 2015, but some very big shocks lies in wait for Mr Cameron and Mr Miliband. A lot of local contests just became hard to call.

Secondly, the Ukip phenomenon is one for Scots to ponder. Earlier this week, a YouGov poll assessed that party's support at one per cent on this side of the Border. Throughout the referendum campaign some No supporters put a lot of effort into the claim that differences in political culture within the UK are irrelevant, or just a little quirk of the system. But when Mr Carswell collects close to 60 per cent of the vote in his constituency, when Ukip and the Tories between them top 50 per cent even in Heywood, facts can't be shrugged aside.

Scotland voted to stick with a Union dominated - for how could it be otherwise? - by voters demanding right-wing answers to right-wing questions. There is precious little appetite for alternatives. Mr Miliband, scrambling to shore up what he imagines to be a core vote, will not ignore that truth. After all, Mr Farage hints future defectors to Ukip could come from Labour as well as the Tories. Scotland made its choice, but its reaction to the consequences has yet to be felt.

That same YouGov poll had the SNP on 42 per cent, Scottish Labour on 29 per cent and the Scottish Conservatives on 17 per cent. Aside from the fact the LibDems are no longer a serious part of the conversation on either side of the Border, this country is, increasingly, anomalous within the politics of the UK. You might even call it foreign. Ukip will certainly pick up votes here in 2015, but it will not upset anyone's calculations.

For Westminster, those are about to become arcane. This counts as the third effect of a pair of by-elections. Who might hold the balance of power after the next general election? Already, a queue of contenders is forming, led by the predictably eager Mr Farage. As he sees it, both Labour and the Tories now have to work out how the Ukip phenomenon might affect their chances of forming a government, not to mention the nature of that government.

Yet judging by his utterances during his party conference, Nick Clegg still expects a role in post-election horse-trading. The LibDems could well wind up fourth in terms of their share of the vote, but the deputy Prime Minister has a core strategy of his own to minimise losses. Whatever happens, he will have many more MPs than Ukip when he makes his offer, once again, of stable, responsible (and so forth) government "in the national interest".

After May 7, things at Westminster are liable to become complicated, not to say messy. Before the referendum and Heywood, Labour calculated disaffected former supporters of the LibDems, the electoral system, and loyalty from those heartlands would be enough to see Mr Miliband into Downing Street. That hope is being shot to pieces, from Glasgow to Greater Manchester.

Mr Cameron has parallel problems. What can he do to calm the Ukip fervour that he has not already attempted? What can he do to placate xenophobes without alienating those who are revolted by the rise of Mr Farage? There are plans, no doubt, for assaults on carefully-selected marginals. There will be schemes to educate English voters in the arts of tactical voting. But the evidence from Clacton and beyond says that a big and growing part of England's electorate is out of patience.

Meanwhile, if the SNP can translate votes into seats - and that will be harder than some of its supporters want to believe - there is the chance of a very big prize. If the main Westminster parties are in a desperate fix, which will then refuse to talk seriously about real home rule for Scotland? Come May, there might be no other choice.