Vasyl Petrovych Holoborodko was a high school teacher whose rant against government corruption went viral and catapulted him into the race for president of Ukraine, which he miraculously won.

He’s also not real, nor was the election. But Volodymyr Zelensky, who stars as Mr. Holoborodko in the hugely popular Ukrainian television show “Servant of the People,” is. And now Mr. Zelensky — whose political party shares a name with his TV show — has a strong chance of winning the actual race for president of Ukraine.

Confusing? Not for Ukrainians, nearly a third of whom voted for Mr. Zelensky in the first round of presidential elections last month and propelled him into Sunday’s runoff with the incumbent, Petro Poroshenko. Being both an entertainer and a populist isn’t so unusual, given the way TV showmen from Italy’s Beppe Grillo to Donald Trump have scored big on popular disenchantment with politics. But there can be little question that Mr. Zelensky, 41, has taken politics-imitating-art to a new level.

Mr. Zelensky’s main card, perhaps his only card, is intense popular disillusionment with the perceived failure of elected officials to eliminate corruption and raise living standards. He has no experience in politics, his platform is a mishmash of vague promises, and he may not be quite the outsider his TV character is. He is linked to one of Ukraine’s murkier oligarchs, Ihor Kolomoisky, whose television channel airs Mr. Zelensky’s show and who has actively promoted his candidacy. Once allied to Mr. Poroshenko, Mr. Kolomoisky is now bitterly opposed to the president.