A 35-year-old businessman without prior political experience has been elected to lead Greece’s main opposition bloc, the left-wing Syriza party

ATHENS, Greece — A 35-year-old businessman without prior political experience was elected Sunday to lead Greece’s main opposition bloc, the left-wing Syriza party.

Stefanos Kasselakis defeated Effie Achtsioglou, a 38-year-old lawmaker and former labor minister, in a runoff contest. Three other candidates had been eliminated in an earlier first round.

With the votes still being counted, Achtsioglou called her rival to congratulate him. Kasselakis had just under 57% of the votes late Sunday with 70% of precincts reporting, while Achtsioglou had just over 43%. About 136,000 Greeks turned out to vote, fewer than in the first round.

Kasselakis, a resident of Miami, was unknown to the Greek public until he was anointed a candidate on Syriza’s at-large list. In Greece’s national elections, 15 of the 300 lawmakers are elected at-large, depending on each party’s share of the votes. At-large candidates can include expatriates. Syriza elected four such lawmakers in the May election and three in June. Kasselakis, in ninth place, had no chance.

Then in late August, a few days after presenting a list of proposals for the party, Kasselakis released a video of just over four minutes recounting his life and his vision. The video went viral, transforming him into a serious contender for the party leadership. Achtsioglou had been the overwhelming favorite.

Nineteen days after the video introduced him to the public, Kasselakis led the first round of the leadership contest, with nearly 45% of the votes to Achtsioglou’s 36%.

To the party’s old guard and left wing, Kasselakis’ candidacy rankled. A rival candidate, former Finance Minister Euclid Tsakalotos, accused Kasselakis of indulging in a shallow, social media-driven “post-politics.” Achtsioglou, who sought to appeal to a broad range of supporters when she was the frontrunner, took up these criticisms.

But rank-and-file party members apparently felt a need for change. Syriza lost power when the conservative New Democracy party won the 2019 elections 39% to 31%. In June’s election, Syriza fell to just under 18%, while New Democracy got over 40%.

Alexis Tsipras decided to step down, forcing the leadership contest.

Despite their youth, Achtsioglou and Kasselakis are older than Tsipras was 15 years ago when he became Syriza leader at age 34, taking advantage of a deep financial crisis. He took the party from around 4% voter support to 35% in 2015.