Political promises to deliver anything that can be accurately measured are always dangerous, and David Shearer is finding out why.
Labour's housing policy, announced in November at the party's annual conference, is called KiwiBuild and it's really catchy.
The nice round figures are easy to remember: 100,000 houses over 10 years to be built for less than $300,000 each.
And there's more. It would be the largest public building project in 50 years, generating $2 billion a year in economic growth through extra jobs and spending on construction materials.
It's Labour's answer to the housing affordability crisis, one of the hottest political issues this year.
If it can be done - and the `if' is really big - it will be great.
Labour will have made a real difference to the lives of thousands of young families who otherwise wouldn't be able to afford their first homes.
Yeah, right, says the government.
The plan is being systematically ripped apart by Labour's political opponents and the numbers are easy to crunch.
There's nowhere near enough available land, National's MPs say.
Figures are produced showing putting up a three-bedroom home for $300,000 in Auckland - where Labour says two-thirds of them will be - is impossible.
And where will all the construction workers come from when the Christchurch rebuild is sucking them in?
Prime Minister John Key says all they're going to get for $300,000 in Auckland is a bedsit or a one-room flat, and Labour's policy is dishonest.
Labour is staunchly defending itself against this tirade of destructive criticism, but with increasing difficulty.
Shearer has been on the back foot a few times, admitting some homes might be small, and he isn't giving the impression that he has all the issues under control.
But the policy is bigger and bolder than anything the government is proposing, and Labour insiders say National's private polling shows voters like it.
They could be right, given the amount of effort that's going into trying to prove it can't be done.
The government knows affordable housing is going to be a big issue at the 2014 election, and it has a problem.
It must, by 2014, be able to show it has made a difference.
So far its efforts haven't been that impressive.
It is leaning on councils, with threats to step in unless they change planning processes and free up land for housing.
It will have a bill in parliament within months amending the Resource Management Act, cutting the red tape that ties up building consents.
A rehabilitated Nick Smith is the new housing minister charged with finding answers.
But there is no plan anything like Labour's, and its claims that councils are the culprits because they won't zone land for development is strongly challenged by Auckland's mayor Len Brown.
The score in this political battle is running about 50-50, but Labour's problems haven't really started.
They will if the party wins the 2014 election and has to put its money where its mouth is.
Even if it hits the ground running it isn't likely to start building houses until around the middle of 2015.
Building costs will almost certainly be significantly higher than they are now and unless the housing market has turned around, so will section prices.
And National in opposition will keep on asking the question: How many houses have you built so far?
When Jenny Shipley was health minister she famously declared: "I will be judged on hospital waiting lists."
Waiting lists increased, and she was judged.
Shearer will be judged on delivering 100,000 cheap houses.