The unexpectedly electric quest to elect a new leader of Greece’s main opposition party, the leftwing Syriza, has intensified as a runoff poll gets under way. Voters began casting ballots at more than 500 polling stations nationwide in a race that has pitted a party cadre and former labour minister, Efi Achtsioglou, against a Greek American entrepreneur, Stefanos Kasselakis. Although an outsider with little experience in politics and practically no ties to Syriza before the last-minute announcement of his candidacy, the 35-year-old Kasselakis emerged victorious in the first round last week, winning 45% of the vote. Achtsioglou, who trailed with 36%, had been the favourite. As many as 180,000 members have the right to participate on Sunday. Electoral stations are scheduled to close at 8pm local time and a result could be announced within hours. Traditional leftists, shocked by the prospect of a shipping executive only recently returned from the US assuming the reins of an alliance whose constituent forces include Marxists and Eurocommunists, are expected to rally. About 40,000 registered members who did not vote last Sunday were able to cast ballots. Kasselakis, now backed by Nikos Pappas, a fellow candidate who won 8.91% of the vote, has campaigned on the basis that with his self-described “better English and finance and business” knowledge, only he can defeat the centre-right prime minister, Kyriakos Mitsotakis, at the next general election. Under its former leader and ex-prime minister Aléxis Tsípras, Syriza was routed in double elections earlier this year. Kasselakis appeared to have won the support of young people but he has been accused of being light on policy, with critics bemoaning his lack of a programme. Syriza cadres have lamented that a man who appeared almost out of nowhere should have come so far in such a short space of time. Unlike Achtsioglou, the Greek American – who was given an honorary position by Tsipras as an expatriate candidate on the state ballot before general elections in May – is not even an MP. Writing in Kathimerini, the columnist Nikos Konstandaras said it was precisely Kasselakis’s mystique that appealed to voters. “Those who vote for him do not consider it a problem that they do not know him,” he opined, likening Kasselakis’s “unorthodox candidacy” to an invasion at the “very top of the party”. He wrote: “They vote for him precisely because they don’t know him.” Kasselakis has broken moulds in other ways: he moved back to Greece with his American husband, Tyler McBeth. This is believed to have worked in his favour, with Syriza voters keen to shake up socially conservative Greece.