He came out the winner by a narrow margin after trailing for most of first hour. He is back in the campaign.
When answering a question from a YouTube viewer on whether police could have more ways of protecting themselves, Mr Key moved into a different gear.
His body language changed, he became more assertive without constantly trying to shout down Miss Clark, and he appeared confident that his tough stance on law and order would be supported by voters.
Miss Clark, wearing a white jacket with black trim - her new election-campaign wardrobe - was as assured as she always is in public debates.
Once Mr Key, wearing a navy suit, white shirt and blue tie, said the words "reasonable self-defence has to be allowed" for shop owners, the debate shifted in his favour.
At times, there was a hint of desperation about both leaders as they were determined to make the last point before an advertisement break.
Mr Key began in an over-energetic fashion, seemingly determined not to make the same mistake as his predecessor Don Brash, who eased back on Miss Clark because she was a woman.
Mr Key and his minders must be breathing much easier after this debate.
The National Party leader had been caught flat-footed by Miss Clark as both parties launched their campaigns on Sunday.
He was again caught off guard on Monday by Miss Clark's universal student allowance announcement.
However, it was clear from the start that he was not going to hang back during the 90-minute (including adverts) debate.
It was polite for most of the time with "John" and "Helen" rather than Mr Key and Prime Minister, as the leaders are inclined to call each other in Parliament.
Miss Clark stuck closely throughout the debate to a carefully crafted script that had worked well since Sunday's campaign launch.
Phrases she used at the University of Otago on Monday turned up again last night.
Mr Key's pet phrases throughout the debate included "fresh start", "fresh ideas", "step change", "under my watch" and "change election".
Miss Clark continually pointed to her experience as a leader of nine years, but that left Mr Key with the chance to criticise the deterioration of the government accounts.
He managed to turn around many questions into those he really wanted to answer.
Mr Key was asked the three different things his party would do to bring a fresh face for government, and to be specific. And, for once, he was.
He would bring in education standards to stop young people from falling through the cracks - something Miss Clark described as "school cert for 6-year-olds"; tax cuts to structurally change the economy; and cancelling parole for the worst offenders.
Miss Clark said her priority was how to lead the country through the worst international financial crisis since the 1930s, feeding the growth engine of the economy.
The economy was her big issue, followed by sustainability and driving growth.
While Miss Clark did not rule out having New Zealand First leader Winston Peters as a cabinet minister in another Labour-led government, Mr Key confirmed he would not deal with Mr Peters.
Both leaders pledged to try to avoid a bitter campaign by focusing on the issues.
Miss Clark said she was campaigning for a full term as the next prime minister, without committing to staying for a full term.
Best lines. -
Mr Key: "The rhetoric is laudable."
Miss Clark: "You might be used to shouting people down at home but you are not shouting me down."
