TAMPERE, Finland, April 1, 2023 (BSS/AFP) - From steering her country into NATO to headlines about partying, Sanna Marin is a modern, feminist leader who has nonetheless proved a polarising prime minister of Finland.

The 37-year-old became the world's youngest elected head of government in 2019, running a centre-left coalition with five women party leaders, four under the age of 35.

She is now fighting to stay in power at legislative elections on Sunday.

Having deftly navigated Finland through the Covid-19 pandemic and led its historic NATO membership application process, Marin is the country's most popular prime minister this century, according to polls.

She has also proved one of the world's most vocal critics of the war in Ukraine and of President Vladimir Putin in Russia, which neighbours Finland.

But her Social Democrats are facing stiff competition from two parties on the right, with all three hovering around 20 percent in the polls in the final days of the campaign.

Before Marin, few people abroad knew the name of any head of government in the small country of 5.5 million.

But in just a few years, she went from relative unknown in Finland to one of the most recognised leaders around the world.

First elected a member of parliament in 2015, she became prime minister in December 2019 and was almost immediately thrown into the global Covid-19 crisis.

"Solemnly, usually dressed in black, she gave very clear instructions, managed to calm the people and create the impression that the government had the pandemic under control," said Marko Junkkari, a journalist at Finnish daily Helsingin Sanomat.

- 'Party Sanna' -

Her popularity peaked during the pandemic, building her reputation as an unwavering crisis leader.

Navigating those difficult years while keeping her clashing coalition in check shaped Marin into a pragmatic, assertive leader.

But she was soon entangled in headlines relating to her private life, polarising her reputation.

In August 2022, leaked social media videos showing Marin partying with a group of Finnish celebrities made news around the world, prompting her to take a drug test to clear suspicions of wrongdoing.

In December 2021, she came under sustained criticism after it was revealed she stayed out dancing until the early hours despite having been exposed to Covid-19.

For some, the criticism overshadowed her political accomplishments. To them, she is "Party Sanna" whose behaviour is inappropriate for a person in her position.

Others defended her right to a private life.

While the controversies propelled Marin to international fame, at home her supporters and haters dug deeper into their trenches.

"These scandals do not change these two groups in any way," Junkkari explained.

University of Turku professor Anu Koivunen told AFP that while the scandals would likely have caused a stir with any prime minister, the fact that Marin is a young woman played a role in the ensuing brouhaha.

Battling controversies about stereotypes or her lifestyle is nothing new for Marin.

- Humble origins -

She grew up in the southern Finnish town of Pirkkala, in a "rainbow, low-income family" in council housing, in her own words.

"My parents divorced because of my father's drinking problem when I was only a few years old," she wrote on her blog.

She was the first in her family to go to university, earning a masters' degree in administrative sciences.

Although Marin's childhood with her mother and mother's female partner did not include "material abundance", it was full of "love and ordinary life," she wrote.

Marin is married and the mother of a five-year-old daughter.

She stands in stark contrast to the Social Democrats' traditional demographic, attracting young voters from the left and rejuvenating the party's image.

"A few years ago, the Social Democrats had more members over the age of 90 than under 35," Junkkari explained.

Young people from other left-wing parties now "have little difficulty" switching to the Social Democrats, said Tuomo Turja of Finland's leading pollster Taloustutkimus.

Often considered a progressive idealist abroad and an embodiment of Finnish values, Marin's policies at home are more pragmatic, sometimes garnering criticism for not being ambitious enough when it comes to climate and human rights.

In October, Marin was forced to apologise for her government's lack of action on indigenous Sami people's rights.