Eddie Barnes: With SNP all fired up, Labour faces a stern test in first by-election since its Holyrood election bruising

THE antennae of Scotland's political classes have been well and truly scrambled by the SNP's unexpected landslide victory in May. Consequently, all bets are off when it comes to predicting the forthcoming Inverclyde by-election, which takes place on 30 June.

By-elections are never good wagers anyway. With local issues often coming to the fore, and the glue of a national campaign absent, it is perilously difficult to predict the mood of local voters. Only a year ago, Labour's David Cairns – whose untimely death has triggered the poll – won the seat at the general election with a majority of more than 14,000 over the SNP. But, a year on, and in the context of a by-election that now counts for little.

Just how little was shown five years ago in the Dunfermline and West Fife by-election. It too was triggered by the death of a sitting Labour MP, Rachel Squire. She, like Mr Cairns, had just won a thumping five-figure majority at a general election. Labour lost the seat by some distance to the Liberal Democrats.

Labour also has recent memory of a its harrowing Glasgow East by-election defeat in the summer of 2007. Like in Inverclyde, it took place just a few weeks after the Nationalists had won power at Holyrood. A trend? Maybe, but then there are also Scotland's two other recent by-elections, in Glenrothes and Glasgow North-East, when Labour easily beat the SNP, the second one humiliatingly so.

Not surprisingly, Labour MPs are deeply cautious. "It'll be a close race," says one of Mr Cairns' friends. Another MP adds: "It's not going to be a walkover, far from it. The SNP know that if they win this, then we are in even bigger trouble than we thought."

Alex Salmond launched the SNP campaign yesterday with a vow to focus on jobs and household bills, and sought immediately to maintain the momentum from the Holyrood result. An SNP victory would "add to that support" and push the case for more "job-creating powers" at Holyrood. Labour, which launches officially tomorrow, is likely to counter this by pursuing a local strategy, headed up by its candidate Councillor Iain McKenzie. The party believes his high profile will contrast with the SNP's Ann McLaughlin who, while Greenock-born was, up until May, an MSP for Glasgow.

What could be key to note is whether the "anybody-but-Labour" vote comes out again. Last month, in Inverclyde as in much of the rest of Scotland, Lib Dem and Tory voters defected to the SNP in their thousands, as they plumped for the SNP over Labour to run the country.

Greenock was one of the few places where Labour still managed to stay afloat, but the margin was a mere 500 votes. Counter to that, being back on a Westminster footing should help Labour hoover up voters who want to bolster opposition to the UK government.

On that subject, it was interesting to learn yesterday that an appearance by David Cameron on the campaign trail is not certain. Don't want to firm up Labour's support, Prime Minister?