Just before Christmas, a delegation of union leaders braved the snow and ice and filed into 10 Downing Street for a rare audience with David Cameron. Organised by the Trades Union Congress (TUC), it was thought to be the first meeting of its kind with a Conservative prime minister in a quarter of a century.

Coffee and mince pies replaced beer and sandwiches around the coffin-shaped table in the cabinet room, as the brothers raised their opposition to the Tory-led government's "dangerous" spending cuts, as well as the proposed reforms to public-sector pensions. "It was perfectly businesslike," Brendan Barber, the TUC general secretary, tells me. "The Prime Minister accepted that we represented a lot of people, that we were a legitimate force in society and that the government should engage with us in a proper way."

But the time for talking is over. Cameron's fiscal vandalism has turned growth negative and pushed up unemployment. In recent months, public opposition to the coalition's cuts has been led by student protesters on Whitehall and the UK Uncut anti-tax avoidance activists on Oxford Street. Some of us had wondered when the seven-million-strong trade union movement would roar into life. Why were the French unionists so much keener to take to the streets than their British counterparts?

Hard target

That is all about to change. On 26 March, three days after the Budget and six days before the cuts start, the TUC is holding a national demonstration - "March for the Alternative" - in central London. (The "alternative" includes a crackdown on tax avoidance, which costs the Treasury up to £40bn a year, and a so-called Robin Hood tax on financial transactions that could raise up to £20bn a year.)

As many as 580 coaches have been booked to carry people from around the country into the capital and trains have been chartered. Could this be the biggest march since 2003, when two million demonstrators took to the streets in protest against the Iraq war? Barber will only say that he is confident it will be "very sizeable"; the TUC has avoided "setting a target". Len McCluskey, the leader of Unite, Britain's biggest union, seems happier to raise the bar. "You need half a million people," he tells me.

But numbers are not enough. If the demonstration is made up entirely of the usual suspects - trade unionists, students and Trots - Cameron and George Osborne will shrug it off and carry on cutting. The Iraq war protests were defined by their breadth and diversity. I remember seeing Telegraph readers marching alongside sellers of the Socialist Worker. Can Barber, McCluskey and the rest build a grass-roots coalition, a broad church, that reaches out beyond trade union and Labour Party members to public-service users, charities and community groups? Will the middle classes turn up on 26 March to demonstrate against cuts to benefits in the same way that they protested against the sell-off of the forests?

The coalition's volte-face on the forests is a reminder of how Cameron can crack under pressure. But Barber is keen to lower expectations. "The idea that we are going to engineer a government U-turn with one event is utterly unrealistic," he says "But is [the government] vulnerable to political pressure? I think it is."

How will the organisers of the rally judge whether or not it has been a success? "I would hope that it will have been a peaceful event," says Barber. "We have worked closely with the police in planning the arrangements." Other union leaders are more suspicious - and more trenchant. "Just as we've sent a message to Cameron and Clegg to keep their sleazy hands off our public services, my message to the Metropolitan Police is: 'Keep your sleazy hands off our kids,'" says McCluskey.

If the march does turn violent, expect the media to have a field day. Anti-union propaganda is back in fashion. Tune in to Radio 4's Today programme and listen to the presenters berating the likes of Barber and McCluskey while deferring to business leaders; flick through the papers and count the number of times that democratically elected general secretaries of trade unions are referred to as "barons", "militants" or "wreckers" - and accused of "holding the country to ransom" by daring to discuss strike action. (Isn't it odd that bankers who threaten to relocate abroad rarely face similar accusations from the press?)

However, in spite of the attempts to denigrate and delegitimise the trade unions and their leaders, the public holds a more benign and trusting view of the brothers. In October 2009, Ipsos-MORI's survey of "public trust in professions" found that union officials, with a 38 per cent rating, were far more trusted than business leaders (25 per cent), journalists (22 per cent) or politicians (13 per cent).

Under the influence

Where does Labour stand? Ed Miliband is walking a tightrope. The Labour leader will be speaking at the rally in Hyde Park on 26 March - but not marching. On the one hand, he needs to harness the energy and enthusiasm of the anti-cuts coalition; on the other, he wants to look "prime ministerial". His aides worry about the accusation that he is a trade union placeman. Senior Labour figures are thus keen to keep some distance from the brothers.

But, as private donations to the party plummet, Labour has become once again much more financially dependent on the unions. According to the latest figures from the Electoral Commission, 88 per cent of all donations to the party in the fourth quarter of 2010 came from the trade unions - up from 36 per cent in the final quarter of 2009. McCluskey, whose Unite union provided nearly a quarter of all donations in 2010, says he has "no intention of giving a blank cheque to the Labour Party as we move forward," adding: "People can accuse me of wanting to buy influence but, as the member of any organisation, you pay a subscription and hope to influence."

On 26 March, however, it will be the political rather than the financial influence of the union movement, as a force for mobilisation and activism, that will be on display. Cameron and his new spin-doctor-in-chief, the former BBC executive Craig Oliver, should batten down the hatches and be prepared for a summer of discontent. Echoing Winston Churchill, McCluskey says that 26 March won't be the end of the campaign against the cuts: "This isn't the end or the beginning of the end. It's the end of the beginning."