Last night the government lost a vote in the House of Lords on one of its flagship policies- capping the value of benefits that one family can receive to £26,000.

You'd expect this to anger, frustrate and irritate Number 10, but I doubt it. In fact it's likely to be quite the opposite. The government will be delighted that it has an opportunity to claim that its opponents have decided that paying someone over the average national wage in benefits is a priority in these austere times. In fact the divisions over last nights vote will set up one of the major fault lines for the next general election, reviving that all too familiar political buzzword - fairness.

Politicians love to claim that they hold a monopoly on fairness, but what is it to be fair? Take the benefit cap; some argue that fairness is ensuring that families out of work do not receive more in benefits than the average working family. Others however claim that it is profoundly unfair to cut the assistance to those at the bottom to pay off the financial follies of those at the top.

There really is no answer to what fair is, but difficult financial times focus the mind and voters are increasingly scared about squeezes on their household income and worry where their money is being spent. This is one of the key motivations for the benefit cap policy, as the government believes that by ending the culture where the unemployed can receive more money than those in work, they demonstrate that they are on the side of the 'striver' and the 'grafter' and that their position is ultimately fair. So far this strategy seems to be working, as recent polls have shown that 75% of voters back a cap on benefits.

Why then if these proposals are so popular would the government be so happy about losing the vote? Often politics is defined by defeating your opponents and the government and in particular the Conservative party will be delighted that Labour, through its members in the House of Lords, have sided with an eclectic mix of Bishops, cross benchers and disaffected Liberal Democrats. Whilst their reasons for voting against the cap may be nuanced and specific- not against the principle of a cap as a whole- this is not the message that will shine through. This allows the Conservatives and to a lesser extent the Lib Dems, to argue that are on the side of the hard working taxpayer and the opposition is on the side of welfare recipients.

This is a powerful story for the Conservatives to tell and is the kind of mood music that voters remember. In times of economic crisis people become more Conservative with their money and are less concerned about equality and more about whether they can afford a holiday.

So where is Labour on this benefits and fairness debate? Not in a good place. I asked one prominent labour blogger today what he thought of his party's political position and his answer was telling: "we are where we are, where ever that is". Their problem has been that they have flip flopped all over the place and have lurched from criticising the cap, supporting the cap, supporting the cap but opposing almost all of it and then seeking to wreck it in the lords.

Not only does this fail the fairness test the government has set out- should someone commuting to work at 6am have to pay more for the person still in bed- but it also fails a basic communication test as well. I always think that if you cannot summarise the basic message of a policy in a 140 character tweet, then you need to look at your message. Labour's position on the benefit cap not only fails this test (tweet me the answer if you think I'm wrong) but seemingly misunderstands the nature of the fairness game - the government not the opposition gets to set the rules.

It is for this reason that all successful oppositions make decisions that are against their instincts and drive their rank and file to distraction. Government is about creating the world as you want it to be, opposition is about reacting to the world as it is. Think Tony Blair sticking to Tory spending plans or Cameron grudgingly accepting the existence of the 50p income tax rate. The reason that oppositions must do this is that government's ability to make policy, enables them to set traps for the opposition that can come back to bite them at the ballot box. For instance, Gordon Brown would have had a hay day if Cameron went into the last election pledging to cut tax on the rich in a period of austerity, in the same way John Major would have battered Labour's economic reliability if they hadn't adhered to Tory spending plans.

But Labour seems to have ignored this core message and risks paying for it at the next election. In 2015 the UK will still be engulfed in the steely grip of austerity and the Conservatives will have a powerful narrative that whilst they have made unavoidable decisions, they have ensured that spending is responsible and it is fair to the average man or woman on the street. Labour on the other hand has only served to re enforce the underlying perceptions (whether fair or not) that they are again helping their 'friends with benefits' and they represent the skivers not the strivers.

Labour activists may think that's unfair, but for 75% of voters it's reality and will be a dividing line up until the next election. Why would the government be so pleased to lose the vote? Because it gives them a once in a term opportunity to claim that elusive but prized concept- fairness.

Follow Ed McRandal on Twitter: www.twitter.com/EdmcRandal