All honeymoons come to an end, and political ones sooner rather than later. So the 18-month period of unalloyed popularity that the Scottish National Party has enjoyed since it came to power has been something of a phenomenon. No setback has disturbed it, no broken promise has been allowed to intrude on the nation's blissful affair with its beloved government.

Now, suddenly, the idyll has been interrupted. The bridal suite has been closed, the candlelit dinner cancelled, the rosy dawn of a new era replaced by the chill wind of rejection. The question now must be: is the SNP's honeymoon to be replaced by a nightmare?

Hitherto the party has been able to brush off criticism from a weakened opposition. The Labour Party, still smarting after its defeat in the Scottish elections last year, has seemed incapable of forming a coherent strategy for the future and has struggled to find convincing leadership. Meanwhile the Liberal Democrats, having lost a role in government, have failed so far to find an identity, and the Conservatives remain in search of their elusive renaissance.

By contrast to this general display of incoherence the SNP has demonstrated self-confidence and disdain in equal proportion. Alex Salmond has been able to deflect criticism by contrasting his party's devotion to the interests of Scotland with the woeful achievements of his predecessors. He has shown that minority government can be made to work if you parade a series of populist measures that do not require legislation, while blaming the lack of anything more substantial on the mean-minded intransigence of other parties.

The strongest Nationalist card has been the remarkable deal reached by the SNP government with local authorities. By persuading them to freeze their council taxes for a year, and possibly for more, Mr Salmond has been able to demonstrate genuine savings for local ratepayers and to contrast that with the increases that the previous Labour regime had introduced. The “historic concordat” he describes it as, deliberately rolling the Rs as he mocks Labour's discomfiture.

There is, however, a flip side to this, and it was exploited ruthlessly at Glenrothes. Some councils have begun to find that, for all the support they receive from central government, the combination of inflation-fuelled costs and frozen taxes has made budgets tough to work within. They have had to introduce charges and cut back services.

Thus it was that the SNP-led Fife council found itself having to introduce some modest charges for home care for the elderly and to seek payments for alarm systems for the disabled. They were means-tested and affected a relatively small number of people, but Labour seized on them as a demonstration that the SNP was punishing the weak and the vulnerable. The party homed in on this with the kind of single-minded ferocity that the SNP once regarded as its own.

It rediscovered its old ability to run a successful campaign and focused on those issues where it could demonstrate the SNP's inability to improve peoples' lives.

The financial crisis played into Labour's hands. Who, it demanded, was better equipped to tackle bank debts and interest rates - Gordon Brown, the iron chancellor, or Alex Salmond, under whom an independent and isolated Scotland would have struggled to survive?

This argument was strong enough to turn the tide at Glenrothes. But that is not the end of it. The issues that won the by-election for Labour are likely to haunt the SNP over the next two years as the party confronts what all governments have to contend with in difficult economic times: councils cutting back on services, hospital boards struggling with huge deficits, promises that simply cannot be funded. Already pledges on free school meals, smaller class sizes and extra policing are having to be reconsidered. And round the corner looms another SNP flagship policy - local income tax - which is beginning to fall apart at the seams.

All this confronts Mr Salmond just at the point where the Labour Party has begun to rediscover its ability to function as a proper opposition. It will be harder for him now to brush aside a party that has a substantial victory under its belt, while at the same time his own image has suffered a damaging blow that Labour will be anxious to exploit.

That may be no bad thing. For too long now the SNP has been able to coast along, buoyed by little more than its own self-approval.

From here on it must learn to behave like a proper government, be tested on its policies, and face a serious opposition. That is good for the country.

It may well be good for the SNP as well.

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