The Government has agreed to a 50/50 sharing of the costs of building the city tunnel with the Auckland Council, starting in 2020 and finishing around 2025. Mr Brown wants to start digging in 2015 and finish in 2021, with the Government not having to pay its share until 2020.
In his letter to Mr Key, the Mayor said that "effectively, council would be underwriting the Government's contribution until 2020 or earlier".
It is a move that leaves the Prime Minister high and dry. If he refuses to play ball with the Mayor and advance the construction date, he'll be handing Labour a free hit in the upcoming election campaign.
In the 2011 campaign, Labour promised to start the city rail link as a matter of immediate priority, splitting costs 50/50 with Auckland Council with a finish date of 2020.
This week, Labour's Auckland issues spokesman Phil Twyford was quick to observe that "Len Brown's offer to kick-start the city rail link is a no-brainer and the Government should grab it with both hands".
He later told me that Labour's 2011 campaign policy stood, and if Labour won this year, "we'll move with speed to get the CRL up and running".
Getting the CRL up and running was a key plank in Len Brown's campaign last year, one that returned him to office with a thumping majority. He's now thrust the issue on to the national political stage, and if the prime minister fails to bend, has ensured it will become a major issue for the third of the nation's electors who are Aucklanders.
Mr Brown says the Government's starting date of 2020 is too late, that by 2021, Auckland's bus networks will be at capacity and peak-time road traffic will have slowed to 7km/h.
He also points to various commercial developments, such as the Downtown shopping centre rebuild, under which the proposed rail link will have to pass as it leaves Britomart Station, as reasons to proceed now.
Still unclear is how Mr Brown plans to fund the Government's contribution "holiday". The sum required could add up, if Mr Brown succeeds in his fast-tracking, to a figure nudging $1 billion.
Reprioritising council budgets would be the obvious solution, and in a recent briefing document to the Mayor, Auckland Transport chief executive David Warburton canvassed this option. But he was quick to demand that his organisation be left out of any such shake-up.
"It is not realistic," he told Mr Brown, "to trade-off AT's capital programme as this is critical to delivery of Auckland Plan outcomes and is generally committed."
A quick look at Auckland Council's forecast capital expenditure up to 2022 suggests that if the $1 billion advance isn't to come from AT, it's hard to see where else it might come from.
Auckland Transport absorbs 47 per cent or $9.4 billion of the 2012-2022 capital spending. Water supply and sewerage come a poor second on $4.9 billion, and "lifestyle and culture" a distant third on $2 billion.
Tolls and similar road user charges are another option, but they are a decision for the Government, and neither major party has shown any appetite for forcing Auckland commuters to pay extra to travel on existing roads.
Then there's borrowing, but that would, in effect, be taxing Aucklanders to pay the interest on a loan taken out on behalf of the Government. So far, Labour is making the best offer - indeed the only offer. Mr Twyford says it will come to the party on a 50/50 basis if it wins this year's election, with the goal of completing the project by 2020.
With a close election being forecast, the ball is now in Mr Key's court.
