UKIP has long demonstrated an ability to win votes in the very special circumstances of a European election. In the last couple of years the party has also begun to claim a presence on many an English local council.
But now it has finally made the breakthrough into the big time at Westminster.
Of course, the circumstances were special. Few imagine Ukip would have won nearly 60 per cent of the vote if anyone but Douglas Carswell had been standing for them in Clacton. But then the fact that Mr Carswell opted to defect in the first place was an indication of the seriousness of the challenge that Ukip now poses.
Meanwhile, there were no such special circumstances in Heywood and Middleton, yet Ukip still managed to win nearly 39 per cent of the vote – far above its previous best by-election performance (28 per cent in Eastleigh).
The party almost toppled Labour from its perch, even though Ed Miliband’s party had called the by-election with almost indecent haste following the death of the previous MP, Jim Dobbin.
All three Westminster parties have been hoping that the Ukip bubble would finally burst as the general election came into view.
Instead, it has now been given the vital oxygen of publicity and credibility.
If the party can also manage to win the seat of the second Tory defector, Mark Reckless, in Rochester, Ukip will have high hopes of turning much of the 13 per cent support it currently enjoys in the polls into votes in the ballot boxes next May.
That, of course, would be a far more modest performance than anything we saw on Thursday.
Voters do indeed, as losing politicians always point out, vote differently in by-elections than they would in a general election. Ukip will likely still find it more difficult to win votes in next year’s contest than they did on Thursday. But 13 per cent could still be enough to have a profound impact on the outcome of next year’s election.
First, Ukip could well displace the Liberal Democrats as the third party in votes. After all, there was little solace for Nick Clegg’s party on Thursday.
In Clacton it recorded its lowest ever share of the vote in a post-war English by-election, while in Heywood its vote fell by a very hefty 18 points.
Meanwhile, given the continuing tendency for Ukip to gain more votes from the Conservatives than from their opponents, such a performance could well cause the Conservatives to lose as many as 20 seats to Labour. Given the narrowness of Labour’s current lead in the polls, that could mean the difference between Mr Miliband having an overall majority and not. But after its narrow escape in Heywood, Labour will worry whether it is at risk of losing seats to Ukip.
Expect increasingly desperate attempts from all three Westminster parties in the next few months to try to put the Ukip genie back in the bottle.
However, it may already be too late.
• John Curtice is Professor of Politics at Strathclyde University