It is four months since British voters expelled Labour and Gordon Brown from power. Only last week did party members get their ballot papers for the election of a new leader.

In May, the advantages of a long campaign seemed obvious. The end of a 13-year stretch in office demanded a period of reflection. So did the election result, which offered Labour a disorienting mixture of relief and rout; bloody defeat with the weird consolation that total annihilation had been widely forecast.

Gordon Brown’s election bid was the biggest failure in that campaign, but not the only one. Nick Clegg’s surge vanished. David Cameron’s pleas for a clear blue majority were ignored.

Voters did not reward any party with a resounding mandate and yet the Liberal-Conservative coalition that took power has wielded it with impetuous energy verging on arrogance. It aims to change fundamentally the relationship between government and citizens. A sustained assault on public spending is the means to do it. It is not clear that the country gave its consent for this experiment, although it will be profoundly changed by the result.

So it matters who becomes the next leader of the opposition, not just for the health of the Labour party but for the vigour of British democracy. The country would be better and more reliably governed if the coalition were routinely forced to defend its decisions against a credible alternative. It has thus far been spared that rigour.

The Labour leadership candidate who has so far landed the most punches on the government is Ed Balls. He has been aggressive in opposition to government cuts and tenacious in harrying the economic analysis underpinning the coalition budget. But while fierce anti-Tory pugilism heartens loyal Labour voters, it does little to woo the rest.

The same deficiency has marked the campaigns of Diane Abbott and Andy Burnham. Ms Abbott has plugged a traditional left agenda with charming candour, while never seriously dispelling the charge that it is a recipe for perpetual opposition. Mr Burnham has campaigned persuasively on causes that fall under his remit as shadow health secretary. But he has not looked like a potential prime minister.

That leaves, as the serious contenders, the brothers David and Ed Miliband. Both are clever, both are passionate in their allegiance to Labour tradition and articulate on the need for those traditions to be renewed for the current political context. Neither is complacent about the scale of the task. A key difference is in how much of New Labour they would repudiate, the project which was once the party’s salvation from an electoral wilderness.

Even then, the differences are often overstated. The leadership campaign has generated caricatures that do neither man justice. Ed Miliband is not a red revanchist; David Miliband is not a slavish Blairite clone.

Nonetheless, Ed Miliband strides more briskly away from Labour’s record in office. The financial crisis, he argues, created a pool of resentment and disillusion with rampant capitalism that should be fuel for a social democratic revival. There are arguments about solidarity, mutual obligation and equality that could not be heard when the City was heaping revenue on the Exchequer and banks were stuffing consumers’ pockets with cheap loans. But when the coalition cuts start hurting, those arguments will be relevant again. It is time, he says, to “turn the page” on New Labour.

David Miliband has a more cautious critique of the government in which he served longer and at a more senior level than his younger brother. He does not flinch from criticising Labour’s unthinking reliance on state power to engineer social change, its tendency to “managerial arrogance” which left ordinary people feeling that the government was not on their side. But he emphasises the need to rebuild a coalition of support from the bottom up and – salvaging the electorally vital kernel of New Labour – a coalition drawn from the broadest possible social spectrum.

David Miliband’s most compelling stump performances have been when evangelising for a Labour-inspired alternative to David Cameron’s “Big Society”; an authentic, grassroots civic revival instead of a Tory stunt to put volunteers where there used to be public services.

Ed Miliband’s promise of a clean break from the Blair-Brown era has much visceral appeal to Labour supporters. But there is strategic purpose in David Miliband’s more nuanced message. He wants to win the leadership with as flexible a mandate as possible. The party must manoeuvre adroitly to challenge the Lib-Con coalition and the best line of attack is still unclear. The older Miliband, alone among the candidates, has stayed alert to how promises made in the cosy climate of a party hustings might sound in a general election campaign, when there are not just tribal Labour fans in the audience. Ed Miliband’s analysis often sounds bolder; David Miliband’s agenda to rebuild the party from its base is discreetly more radical.

If the job on offer was for someone simply to energise Labour supporters, reviving their morale and leading them into a principled charge against the coalition, Ed Miliband might be the best candidate. His advocacy of traditional Labour virtues is more naturally fluent, his rhetoric more ardent. But those skills can be deployed from anywhere on the frontbench.

By contrast, there is a breadth and subtlety to David Miliband’s campaign that elevates him above his rivals. He is unquestionably loyal to the Labour tradition, but loyal also to the politics of winning general elections.

The party is uncertain of its chances of recovery because it is uncertain how strong the partnership between Mr Clegg and Mr Cameron will prove to be. Like many a shotgun wedding, it might suddenly unravel in unseemly public rows. But it might also be a lasting union. The coalition leaders hope to build a political empire based on shared liberal values of individual enterprise and personal responsibility. They would like to see Labour consigned to the margins as the party of spendthrift welfarism and state meddling.

The Labour party would be wise to choose a leader who has the intellectual agility and political experience to meet that threat. The combined skills of the Miliband brothers, working in concert, will be essential. For the top job, David Miliband is the better candidate.