Volodymyr Zelenskiy, a popular comic actor who's leading in the polls far ahead of President Poroshenko, said "we have clearly chosen our path to Europe."
But he emphasized the need to secure a solid majority backing the country's prospective membership in the alliance.
"It's obvious that NATO means security and the military's high level, but I want to unite the country," he told RBC Ukraine website.
Zelenskiy added that he would appeal to people living in the mostly Russian-speaking eastern Ukraine to dispel their negative view of NATO. "I will record daily video appeals to the eastern regions of Ukraine to tell them that NATO isn't a beast that is going to swallow you," he said.
Poroshenko has pushed for Ukraine's membership in NATO and the European Union, though neither alliance is willing to put the country on a track to membership any time soon because of its economic woes, rampant corruption and the five-year conflict with Russia-backed separatist in eastern Ukraine.
The fighting in the east erupted in April 2014 weeks after Russia's annexation of Crimea and the ouster of Ukraine's former Russia-friendly president.
Zelenskiy vowed to make ending hostilities in the east his top priority. In Thursday's interview, he pledged to reach out to people living in the rebel-controlled areas to again make them feel part of Ukraine.
Poroshenko's government has introduced a sweeping transport blockade of separatist territories, making it extremely difficult for the people living there to receive pensions and other social payments.
"I very much hope to make these people feel that Ukraine needs them just as much as they need Ukraine," said Zelenskiy, who won nationwide fame for playing the nation's president in a popular TV sitcom. "It would be necessary to take a lot of humanitarian steps."
Zelenskiy also said he would push for an early parliamentary election. Ukraine's Supreme Rada is controlled by Poroshenko's ruling party, and the next parliamentary election is due at the end of October.
Zelenskiy said an early parliamentary election of is part of his idea of a "renewal" of the country's ruling elite, but conceded it may take months to make that happen.
The latest opinion poll released Thursday showed the comedian maintaining a massive lead against Poroshenko. Supporters see Zelenskiy as a man who could tackle the nation's economic woes and corruption, but his opponents point to his lack of political experience.
The survey, conducted by the Rating agency, shows Zelenskiy with 58% support while Poroshenko has just 22%. The poll of 3,000 people had a margin of error of 1.8 percentage points.
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