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The joint Helensville/Te Tai Tokerau candidates meeting at the Kumeu Baptist Church will be the only local meeting Mr Key attends and his arch rivals Laila Harre, the Internet leader, and Hone Harawira, the Mana leader, had hoped to use it as a platform to tackle him.
Ms Harre had even brought along her own security in case things got out of control. The police were present, too.
Ms Harre managed to get in a swipe at Mr Key, but she did it subtly.
So it was the Prime Minister who ran foul of the rules first when he began talking about the superannuation age and Labour's policy of free GP visits for over 65s. It took convener Holly Ryan a while to interrupt and give him his first warning.
When he tried to argue, she said, "Do you want to leave now?"
Ms Harre's first dig was when she recalled her own political beginnings during apartheid protests in the 1980s.
"And I remember which side I was on!" she said, referring to Mr Key's memory failure on that topic. Asked about a local roundabout, she said: "I'll leave that to your local MP, who won't have the pressures of high office after September 20."
It earned her a boo from many in the room - the locals had earlier wanted to know what she would do for Helensville, given she was a local candidate. Her answer: "Change the Government."
After the debate, Ms Harre and Mr Harawira said the Prime Minister should have been evicted.
"He broke the rules time and time again," Mr Harawira said.
Ms Harre said a different set of rules seemed to apply to Mr Key. "I thought it was one strike and you were out. He had three strikes and he was still in."
During the debate, the Prime Minister was asked about religious education in state schools and said it was up to boards of trustees, not the Government.
The other candidates also managed to get a word in before the largely good-natured crowd.
After Mr Key spoke about motorway developments, Mr Harawira questioned priorities such as spending on roading when children were living in poverty.
"No offence to the people from Kumeu, who now have a roadway to the airport, but seriously, we could have done away with the tunnel, we could have done away with the Holiday Highway."
The best one-liners went to Act candidate Phelan Pirrie and Labour list MP Kelvin Davis.
Mr Pirrie was asked about charter schools. "They're all doing very well, except for the one run by a trade union."
Mr Davis was asked if raising the top tax rate would send highly educated people overseas. He replied that it wasn't taxes that drove people overseas, "it's low wages".
Even Independent Penny Bright got a good-natured cheer after having to battle to get into the debate.
But it was always going to be tricky taking Mr Key on in his home turf, where he has a 21,000-vote majority.
The other candidates learned a lesson: the early bird gets the worm.
The meeting was so hotly anticipated that the church hall and an overflow hall were full an hour before the debate started - leaving many candidates' supporters out in the cold.
Judging from the applause for Mr Key, National voters got there early.
Other candidates included Corie Haddock for Labour, Kennedy Graham and Marama Davidson for the Green Party, Andrew Williams for New Zealand First, Te Hira Paenga for the Maori Party, Stephen Berry for Act and Deborah Dougherty for the Conservative Party.