Former Ukrainian President Victor Yushchenko says he doesn't think Putin will resort to nuclear option

Though threatened by the Kremlin for decades, for former Ukrainian President Victor Yushchenko, the Russian invasion of his country, he says, is a tragedy that Ukrainians didn't envision becoming reality.

The 68-year-old told ABC's "Nightline" the situation has united his people in a way that caught Russian forces off guard.

"Russia has never in its history encountered such determination, such a high democratic spirit and spirit for freedom," Yushchenko told ABC News in a video interview from an undisclosed location in Ukraine. "In terms of spirit, [of] understand[ing], a totalitarian Russia cannot defeat Ukraine."

Yushchenko, who served as the country's president from 2005 to 2010, said Ukraine has developed a democracy for the last 20 years despite internal bickering.

"Putin is in an absolute, extreme isolation and that is why, every day, his reputation as the Russian president declines and his political beliefs, including nuclear inclinations, are devaluing fast," he said.

By comparison, Ukrainian President Volodymyr Zelenskyy has worked hard to consolidate the Ukrainian nation, Yushchenko said.

"He’s doing very important work. It's possible,” he added, “that we've not been this united in 30 years. Tragedy and pain can unite."

Zelenskyy has continually released televised updates as Russian forces continue to launch missile attacks and advance on the ground into the country. Zelenskyy has said he is Russia’s number one target in this war, his family, the second.

Yushchenko is no stranger to threats against his own life. He survived a dioxin poisoning in 2004 when he ran against a Kremlin-favored candidate for the presidency.

Yushchenko's face was memorably heavily disfigured for years and some Ukrainian officials alleged that the Russian government was involved. The Kremlin has never officially responded to those allegations.

Yushchenko said his country is appreciative of the steps taken by the U.S. and Western allies to help the Ukrainian people, including sanctions and aid, but he reiterated calls for a no-fly zone.

The former president said "the Achilles' heel of the Ukrainian defense" is strategic Russian airstrikes.

"When we're talking about what Ukrainian soldiers want to have on the war field, any soldier's first sentence would be 'close airspace over Ukraine,'" he said.

ABC News' Mary Marsh and Karin Weinberg contributed to this report.