Andrew Grice: Voters recognise self-interest when they see it

Inside the election

"All those prawn cocktails for nothing; never have so many crustaceans died in vain." In one of the best political gags of recent times, Michael Heseltine ridiculed Labour's "prawn cocktail offensive" in the City as it tried to shake off its anti-business image.

The year was 1992, and the pre-election offensive was being led by Neil Kinnock and his shadow Chancellor, John Smith. Coincidentally, Labour was committed to a rise in national insurance contributions, the issue dominating this year's campaign. Then, the proposed rise in NI and a planned 50p rate of income tax deterred many in the City and in business from succumbing to Labour's soothing noises.

In 1997, Tony Blair and Gordon Brown eventually won their confidence by ditching the symbolic 50p tax rate, pledging to stick to the Tories' spending plans for two years and offering the trade unions "fairness, not favours". After winning power, New Labour handed control of interest rates to the Bank of England and extolled the virtues of "light touch" City regulation – the latter proving to be a mistake in the financial crisis, Cabinet ministers admit.

Now the wheel has come full circle: another proposed rise in NI seems to have alienated the business community. Yesterday, 54 Scottish company bosses added their names to the 81 who had already come out against Labour's "tax on jobs". To make matters worse, there were plenty of headlines this week saying Labour was "at war" with business, after Mr Brown and Lord Mandelson said the bosses had been "deceived" and some of them hit back with knobs on.

The endorsement by so many business leaders – no doubt after a bit of cajoling by the eager beavers at Conservative Campaign Headquarters – validated the Tory attacks on Labour.

Third party endorsements are even more valuable at a time when the reputation of politicians is so low. Tory officials are delighted with the "branding" that the business leaders gave them, not least because the firms included trusted brands like Sainsbury and Marks & Spencer.

Yet all is not quite what it seems. The business figures do not want to see their firms' NI bill rise. The Tories offered them the choice between that and nebulous attack on "government waste" and – surprise, surprise – they preferred the latter.

Although they were not saying "vote Tory", some bosses no doubt judged there was safety in numbers and that it would be in their commercial interests to be on good terms with the party most likely to form the next government. Some are natural Tories whose relationship with Labour was just business, not personal. Indeed, some of the 23 original signatories of the original letter to the Tory-supporting Daily Telegraph may pop up as new Tory peers if Mr Cameron wins the election.

I suspect that the company bosses who attacked the NI rise feel just as angry about Labour's decision to bring in a 50p top rate of tax on earnings of over £150,000 a year – another echo of 1992 – which took effect last week and will hit most, if not all, of them personally. They wouldn't shout that from the rooftops: attacking a "tax on jobs" is much more likely to win public sympathy.

Despite the headlines, the marriage of convenience between Labour and business is not over. Ironically, business figures have been, for the most part, happy with interventionist measures such as the car scrappages and "time to pay" (tax) schemes the Government adopted during the recession. It is even more ironic that Lord Mandelson has attracted their fire over NI.

Most bosses he meets seem to regard him as a good Business Secretary who "gets" his brief. The thing they bend his ear about is the need to tackle the £167bn deficit in the public finances. Which, of course, is precisely what the NI increase is designed to do, and which was the Tories' top priority until they veered off down the populist route of reversing most of the NI rise. This gives Labour strategists a ray of hope of seizing the moral high ground on the deficit.

They sensed a turning point yesterday when the spotlight fell on the job cuts implied by the £6bn of "efficiency savings" the Tories need to halt the NI increase. Most bosses would probably warm to Lord Mandelson's "industrial activism" strategy – not a return to Old Labour's "picking winners" but a rebalancing after the dangers of a free market approach were exposed by the crisis and a way of nurturing industries that will create jobs in an economy less dependent on financial services.

How damaging is the bosses' intervention in the real world? Although the Tories were careful not to jump into bed with the Royal Bank of Scotland and Lloyds, the gap between salaries at the top and the bottom has widened. Public anger over the bail-out of the banks after they created the crisis may reduce the value of the business currency on the political markets. Voters are not stupid: many will recognise the self-interest the bosses have in avoiding a tax rise.

People would be even less sympathetic to the company chiefs if they knew how miffed they were about the 50p income tax rate. The "business vote" might be less important than the Tories hope and Labour fears.