- Latest poll put Labour ahead of incumbent National Party
- New Labour leader Jacinda Ardern only took over the party at the start of August
(CNN)"Bill English, why are you losing?"
That was the opening question in Thursday's New Zealand election debate, and one many supporters of the incumbent prime minister may be asking themselves as his National Party suffered its worst poll showing in 12 years.
In a development no one would have predicted only a month ago, a resurgent Labour Party has swept into first place in a new poll conducted for state broadcaster TVNZ.
The poll, released Thursday, put Labour up six points at 43%, with National at 41%.
According to CNN-affiliate Radio New Zealand, Labour's average in the three most recent polls was 39%, compared to 41.8% for National. Trends show Labour is increasing support, while National has been steadily dropping, the broadcaster reported.
New Zealanders go to the polls on Saturday, 23 September 2017. The party with a majority in the 120 seat House of Representatives forms the government, meaning only Labour and National have a realistic chance to take power without forming a coalition.
English -- who replaced former National leader John Key in December -- called the election in February, as Labour was seen to be struggling in the polls.
But, just as UK Prime Minister Theresa May was stunned by her Labour opponent, Jeremy Corbyn, who overcame the Conservatives' huge polling lead to steal their majority and force them to form a coalition with the far-right DUP, English has found a far stronger opponent than he expected.
Jacinda Ardern, a 37-year-old three-term member of parliament, was elected Labour leader on August 1. Since then, her party has surged in the polls, leading the local press to coin the term "Jacindamania."
If elected, Ardern would become New Zealand's third female prime minister, and the second-youngest, after Edward Stafford, who become Premier in 1856. He was also 37, but born later in the year.
The decision by Ardern's predecessor Andrew Little to give up the leadership in the run-up to an election was an "unprecedented" move, said Grant Duncan, an expert on New Zealand politics at Massey University.
While there may have been an underlying desire for change from the three-term National government, the only likely alternative under Little was an unsteady coalition of Labour, Greens and New Zealand First, Duncan said.
"Jacinda seems to have created momentum for people shifting their growth into the Labour Party," he added. "Suddenly an alternative Labour-led government really is looking viable."
While his predecessor Key was very popular, the 55-year-old English has struggled to match Ardern's charisma and energy, Duncan said.
The two leaders clashed head-on for the first time Thursday, in a televised debate that focused on policy issues.
English criticized Labour's policies for being "vague and confusing" and pointed to his government's record on the economy, which saw modest growth in the most recent quarter.
"National will look to pound their perceived advantage on the economy until election day," analyst Jon Johansson wrote for Radio New Zealand this week.
But English was attacked by Ardern over housing prices, a key election issue.
New Zealand has the fastest rising house prices among major developed countries, according to Economist data, with a 13% increase in the past year.
Labour has promised to built 100,000 new affordable homes if elected, and Johansson said "National's nine years has not delivered for people (Ardern's) age and younger."
Making predictions for how opinions will change in the last few weeks of the campaign is even more difficult than normal, Duncan said.
"No one thought we were going to be in this position now," he said. "All bets are off."