One recent poll showed Labour slightly ahead. Since Ms. Ardern’s rise, the party’s projected share of the vote has jumped by around 20 points.
“In the last two elections, it’s been relatively clear that the governing party would be the National Party,” said Andrew Geddis, a professor at the law school of the University of Otago. Now, he added, “we might finally have a real election.”
A Steady Climb
A rising star in Labour since joining Parliament in 2008 as its youngest sitting member, Ms. Ardern is unconventional, accessible and ambitious.
In an interview with The New York Times last month, she answered questions carefully but also joked about the stress of her new position. The number of days until the election, she said, is written in her diary “like a horrific countdown to Armageddon.”
On Twitter, she has 80,000 followers, more than anyone else in the country’s Parliament. She once performed as a D.J. at a music festival. Her most scandalous moment in public life so far involved her attempt to install her own bathroom rather than hiring a professional to do it, drawing disapproval from a national plumbers’ board.
Her colleagues describe her as intensely focused. She grew up as a Mormon in a rural town working part-time jobs and earned a degree in communications from the University of Waikato. She worked her way up the Labour Party ranks, and along the way, she said, she discovered a passion for child welfare and economic equality.
Grant Robertson, the party’s finance spokesman and a close friend, recalled sharing a small office with Ms. Ardern in 2005 when they were both advisers to Helen Clark, the prime minister at the time who was the country’s first elected female leader.
Ms. Ardern was “unflappable,” Mr. Robertson said. “She was a person that took seriously everything she did in her working life, and I didn’t get a sense she was there to aim for a particular position. She was just doing the job that had been put in front of her.”
In March, she was elected deputy to the Labour Party’s leader, Andrew Little. But after polls showed the party was set to fail with Mr. Little at the helm, he made a surprise move: He stepped aside and named Ms. Ardern as his replacement.
When Ms. Ardern found out about the resignation, she was in a car on the way from the airport. An hour later, she officially had the top spot.
There was so little time to transition that she had to “get on with it,” she said.
It seems to be working. Labour and the National Party are statistically even in the polls.
Some political experts question whether Ms. Ardern has the experience needed. She has spent her career in the opposition and has not championed a bill in Parliament.
“The question will be whether she can add the substance and steel that New Zealanders may look for in a leader of their country,” Professor Geddis said.
Ms. Ardern said that she relishes the challenge. “I’m actually enjoying the chance to demonstrate that I have in my heart and always will be a policy wonk,” she said.
Her campaign website includes in-depth proposals for free university education and a call for longer limitations on rent increases, among other things.
‘Lipstick on a Pig’
Ms. Ardern is no stranger to the focus on her appearance and gender; once a panelist referred to her as “a pretty little thing.”
But only a day into her leadership, a television commentator argued that employers had the right to ask women about their childbearing plans, prompting her steely response, which was applauded by women around the world.
There have been other incidents as well. Earlier in the campaign, Paula Bennett, New Zealand’s deputy prime minister, said that Ms. Ardern, unlike the prime minister, Bill English, did not have the substance and “the kind of brain to pull this country together,” which prompted another backlash.
Then Gareth Morgan, a leader of the Opportunities Party, said on Twitter that Ms. Ardern needed to prove she was more than “lipstick on a pig.”
The comment was widely condemned, including by the prime minister. Mr. Morgan defended himself, saying that the phrase was a “euphemism for a meaningless face-lift or makeover.”
Ms. Ardern said that she had tried to walk a fine line, standing up against sexism while also trying to share personal details that might help people understand who she is.
“I think probably one of my first interviews in Parliament, someone questioned me at length about my marital status,” she said.
She has decided to speak openly — she often talks about her long-term partner, Clarke Gayford, who hosts a travel and fishing TV show — because she thought it might help other women juggling decisions about their career and because she believes that people want to get to know their representatives.
“They want to know a bit about them — what kind of humans they are, what kind of values they have,” she said.
And in New Zealand, she added, even the nation’s most powerful figures are expected to be down to earth. She said that people felt comfortable approaching her about the country’s politics, even while she was shopping for groceries.
“I can tell you a number of times where I’ve been standing in the aisle looking at rows of muesli bar boxes and people have come up to give me their opinion or to ask for help,” she said. “And that’s just the way it is here.”
Still the focus on her appearance has been a cause of concern for many women. When Ms. Ardern addressed an environmental conference in Auckland last month, some women in the crowd said they were happy to see the conversation shift toward policy, not appearances.
“I was very inspired by the environmental policy ideas that she put forward,” said Ann Brower, a senior lecturer in environmental policy.
“I have to say,” she added, “I wish people would stop saying how pretty she is.”
Because of an editing error, an earlier version of this article assigned an incorrect distinction to Helen Clark. She was New Zealand’s first female leader to take office after a general election, not the first woman to hold the position. (That was Jenny Shipley, who became leader after her predecessor resigned.)
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