Finland's most popular president in more than three decades, the 69-year-old who campaigned as an independent has skillfully shifted the EU member state closer to NATO without antagonising Russia, with whom the Nordic country shares the longest border in the bloc.
Finns "want stability and don't want change right now," Juhana Aunesluoma, research director at the University of Helsinki Network for European Studies, told AFP.
The latest opinion polls credit Niinisto with between 51 and 63 per cent of votes, losing ground but still far ahead of the seven other candidates. His main rival, Pekka Haavisto of the Green party, is seen garnering around 13-14 per cent support.
If he is elected for another six-year term with at least 50 per cent of votes in today's first round, it would be a first since Finland introduced a two-round presidential election by popular vote in 1994.
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If no candidate gets 50 per cent, then a second round of voting will be held on February 11.
As Finland's head of state and supreme commander of the armed forces, the president shares responsibility with the government for defence and foreign policy, though not EU affairs.
The two leaders played an ice hockey match in 2012 and attended an opera together last year as part of Finland's 100th anniversary of independence after the fall of the Tsarist Russian empire.
"Niinisto's strategies and tactics have been rather successful, especially handling Putin," Aunesluoma said.
"People get the sense that he has the capacity and tools to cope with the challenges."
At the same time however, Finland, a Russian Grand Duchy from 1809 to 1917, has forged increasingly close ties with the United States and NATO, of which it is not a member unlike the Baltic states.
"One of the central goals of Finland's foreign and security policy is to avoid getting pulled into an armed conflict," Niinisto told Finnish defence forces in a speech earlier this month.
Teivo Teivainen, professor of world politics at the University of Helsinki, said Niinisto's "ambiguity" on NATO membership "was a successful strategy" during the election campaign as he has not disgruntled voters who are either for or against joining the alliance.
Finnish and Russian defence officials announced last year that they would set up a 24-hour hotline to avoid any "misunderstandings".
Born into a working-class family in the southwestern Finnish town of Salo in August 1948, Niinisto -- the youngest of four children -- became a lawyer before entering politics as a member of the conservative National Coalition Party.
He served as justice minister in 1995-1996, before taking over the finance portfolio until 2003.
Highly-publicised relationships and a life scarred by tragedy have given Niinisto a human image among Finns.
The father of two sons lost his first wife in a car accident in 1995 and narrowly survived the 2004 tsunami in Thailand by climbing a tree with his youngest child.
Following a high-profile affair with a former beauty queen-turned-MP, the couple got engaged in 2003 but broke up the following year.
He married his second wife, the Finnish poet Jenni Haukio -- 29 years his junior -- in 2009. The couple announced in October they were expecting a child in February, hitting a soft spot among voters.