The centre-left leader had been in office barely 18 months on March 15 last year when a self-avowed white supremacist opened fire at two mosques during Friday prayers, killing 51 and injuring another 40.
Faced with a crisis unprecedented in New Zealand's modern history, Ardern rose to the challenge with a mixture of compassion and decisive action.
She offered support for New Zealand's Muslims, rejected the shooter's ideology, immediately moved to tighten gun laws and launched a global initiative to curb online extremism.
Ardern's personal popularity rating peaked at 51 percent shortly after the shootings and her Labour Party briefly reached similar levels, setting her on a path to reelection in polls set for later this year.
But the September 19 vote is now looking uncomfortably tight for the 39-year-old, with the centre-right National Party edging ahead five points in opinion polls to 46 percent.
Labour's support has not fallen off a cliff but appears to have plateaued amid perceptions it has failed to deliver on issues such as affordable housing and reducing child poverty.
It joined forces with two minor parties to form a government after the 2017 election, but one of its coalition partners, New Zealand First, is currently flatlining in the polls.
Professor Stephen Levine, a political scientist at Wellington's Victoria University, said Ardern -- like US leaders John F. Kennedy and Barack Obama before her -- was more popular overseas than with domestic audiences.
"Internationally, people are looking at the big picture, not the day-to-day details," he told AFP, adding: "It is by no means inconceivable that Jacinda will be a one-term prime minister."
Massey University politics specialist and associate professor Grant Duncan said many international observers would be baffled that a charismatic, widely praised leader was facing such an uncertain future.
"For an outsider looking at New Zealand, we have this amazing prime minister, she's ticking so many boxes but guess what, she might not be prime minister after the next election," he told AFP.
Ardern won office on a wave of "Jacinda-mania" after being thrust into the Labour leadership just seven weeks before the 2017 election.
She made headlines again a year later when she became only the second prime minister in the world to give birth while in office -- after Pakistan's Benazir Bhutto in 1990.
But it was the way she handled the horrors of Christchurch that defined Ardern's image.
"This is one of those crucial moments when a leader has to rise above politics to really represent the country symbolically -- and she nailed it," Duncan said.
"She focused on the victims, not the perpetrator. What she said was healing and the nation needed healing at that time."
The moment that most resonated globally was when Ardern donned a headscarf while comforting victims' families after the shooting, later saying it was a spontaneous gesture of respect to the Muslim community.
She was among the bookies' favourites for last year's Nobel Peace Prize and just this month featured on the cover of Time magazine accompanied by a positive article.
But Levine said Ardern remained a centre-left leader in a country which, despite its progressive reputation, has a large conservative base. He said this meant Ardern's appeal -- which lifted Labour's polling by almost 15 points after she became leader -- came with in-built limitations.
"For a lot of people in this country, it's part of their identity to be National or Labour - these are tribes really and that's just the way it is," he said.
Duncan said many National supporters admired Ardern's handling of the Christchurch shootings but it would not sway them at the ballot box.
"They're not being budged because we have this celebrity prime minister, even if they appreciate what she did with the mosque shootings," he said. "That doesn't cut it for them."
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