Finns will elect a new president on Sunday in the first national election since the country joined NATO, choosing a leader who will be critical to shaping the country’s role in the alliance at a time of increasingly fraught relations with Russia.

The election might typically gain little notice beyond the borders of the sparsely populated northern European country of 5.6 million. But Finland, the newest member of NATO, shares the longest border with Russia — some 830 miles — and its politics have taken on special interest to its European and American allies as the geopolitical order shifts.

U.S. power is being challenged by Moscow and Beijing, and Europe is grappling with its largest land war since World War II. At the same time, the American commitment to aiding Ukraine looks increasingly in doubt, and an unpredictable American presidential election looms.

Finland’s president is responsible for foreign policy, and whoever wins will bear chief responsibility for steering the country through a changing world.

“The future president is going to have an impact on what kind of a NATO country Finland will be in the future,” said Jenni Karimaki, a political analyst at the University of Helsinki. “NATO membership is one of the things creating interest in these elections — and of course, the overall global political situation.”

Finland’s decision to join NATO was a sharp break with decades of nonalignment, and the risks and responsibilities of the country’s new place in the world dominated the campaign over who should succeed the popular Sauli Niinisto, whose second six-year term expires in March.

The two candidates who made it to the runoff on Sunday — Alexander Stubb, of the center-right National Coalition Party, and Pekka Haavisto, of the center-left Green League — have both strongly supported the decision to join NATO and take a hard-line view on Russia. The differences between them have been mostly stylistic.

Mr. Stubb, a former prime minister who had the most votes in the first round, has played up his security credentials.

“I’m as hawkish as the best of them, there’s no question about that,” he told The New York Times.

He said countering Russia had become more difficult in an era of hybrid warfare. There has been a surge in cyberattacks, some of which Russian hackers have claimed responsibility for.

Among the most concerning issues to voters has been a sudden spike in asylum seekers crossing into Finland over the Russian border, which many in Finland view as a signal from Russia in response to its NATO membership. Moscow had warned there would be “countermeasures” for Finland joining NATO.

“The line between war and peace has been blurred,” Mr. Stubb said. “Russians are very good at hybrid warfare.” He added: “They will do everything to intimidate or destabilize Finland and especially public opinion. But so far, they have failed totally.”

Mr. Haavisto, who was foreign affairs minister from 2019 to 2023, has used his credentials as one of the main negotiators for Finland’s entry into NATO to show that his stance on Russia is equally tough. But he has also shown a wariness for the most hawkish positions. His identity has been shaped by years as a peace negotiator for the United Nations, Finland and the European Union.

The difference in the two candidates’ approach was made memorably clear during one of the debates. Asked whether they would answer a congratulatory call from President Vladimir V. Putin of Russia if they won the election, the two split: Mr. Stubb said he would not. But Mr. Haavisto said he would.

There are only a few other positions that genuinely set the candidates apart, such as their stance on nuclear weapons. Mr. Stubb has said he would be willing to allow the alliance to transport its nuclear weapons on Finnish territory, while Mr. Haavisto said he would not.

The question remains hypothetical, however, as current Finnish law prohibits nuclear weapons on Finnish territory, and the president cannot legislate.

Mr. Haavisto has traveled the country holding listening sessions at gas station centers, a common hangout spot in smaller towns across rural Finland.

He has also held several campaign events that he D.J.ed himself using his nickname DJ Pexi, playing everything from the Beatles to Belgian punk. One of his final campaign events was a concert in which several famous Finnish musicians played.

“Voting for Pekka Haavisto is important to me, because I want to hold on to the one last bit of peace in an increasingly belligerent world,” said Eino Nurmisto, a social media influencer who attended the concert.

Mr. Stubb, an avid athlete, began the second round of his campaign with a walk through central Helsinki, and has held cross-country skiing campaign events. He also opened a string of cafes around the country, for voters to stop in and escape the frigid temperatures with coffee, sweets and campaign paraphernalia.

“We are living in times that will be very important to Finland’s future,” said Claes-Henrik Taucher, warming up at a cafe in Helsinki with a coffee.

Beyond Russia, there is another concern, across the Atlantic: What is in store for Finland’s NATO membership should Donald J. Trump, an outspoken critic of the alliance who has even suggested the United States leave it, win the presidential election in November?

“The whole decision of joining NATO banked on the idea that the U.S., the Americans, are here to stay and that U.S. commitment is long lasting,” said Matti Pesu of the Finnish Institute of International Affairs. “If the U.S. decided to weaken its commitment, it would be a huge irony, and it would weaken the deterrence value of Finland’s NATO membership.”