"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 insurance 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 national insurance seems to have alienated the business community. On Friday, 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 last week saying Labour was "at war" with business, after Brown and Peter 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 HQ – 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 such as Sainsbury and Marks & Spencer.

Yet all is not quite what it seems. The business figures do not want to see their firms' national insurance bill rise. The Tories offered them the choice between that and nebulous attacks 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 Cameron wins the election.

I suspect that the company bosses who attacked the insurance rise feel just as angry about Labour's decision to bring in a 50p top rate of tax on earnings 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 scrappage and "time to pay" (tax) schemes the government adopted during the recession.

It is even more ironic that Mandelson has drawn their fire over national insurance. 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 £167 billion deficit in the public finances. Which, of course, is precisely what the insurance 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 rise. This gives Labour strategists a ray of hope of seizing the moral high ground on the deficit.

They sensed a turning point when the spotlight fell on the job cuts implied by the £6 billion of "efficiency savings" the Tories need to halt the insurance increase. Most bosses would probably warm to 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? 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 may reduce the value of the business currency on political markets. Voters aren't stupid: many will recognise the bosses' self-interest 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.