Stefanos Kasselakis, the Greek American shipping investor and former banker who burst into Greek politics over a year ago, has said he is leaving leftwing Syriza, the country’s main opposition party he had led, and forming his own.

Kasselakis announced he was leaving Syriza on Friday after a two-day party congress refused to consider his leadership candidacy in a contest that will take place later this month.

Kasselakis, 36, has lived most of his life in the US and only entered Greek politics in May 2023 as a candidate in a national election. Syriza suffered a disastrous double defeat at elections held in May and June of that year. He unexpectedly entered the leadership contest to replace longtime leader Alexis Tsipras and triumphed in September 2023.

His main opponent, Efi Achtsioglou, and several of her supporters left the party in December 2023, forming a new organisation, New Left. Many of the Syriza members who had supported Kasselakis soon soured on him, accusing him of not being a true leftist. Kasselakis did not endear himself with his confrontational style and open disdain for the elected party organs.

In September, Syriza’s central committee ousted Kasselakis from his post after a motion of no confidence. He contested the legitimacy of the vote and was set to run in the leadership race but was stopped by the party’s central committee.

Along with Kasselakis, four MPs also left the party on Friday and Saturday. This has left Syriza with 31 MPs in the 300-member Greek parliament, equal with the socialist Pasok party.

Syriza, once dominant, has been in disarray since its resounding defeat by the conservative New Democracy party in the 2023 elections.

Already in opposition since 2019, Syriza had hoped to regain power. Instead, it was roundly disavowed, falling in the most recent election to 17.8%, compared with New Democracy’s 40.6%, a result that led to Tsipras to announce his resignation.

Kasselakis’ style, based on social media presence and charisma, and his past as a former Goldman Sachs employee and shipowner, rankled with old-school leftists, who were also incensed by some of his positions, such as advocating stock options for employees, and his overall ideological vagueness.

They openly derided him for his “post-politics” and one prominent party person even compared him to former US president Donald Trump and comedian Beppe Grillo, founder of the populist Five Star party in Italy.

Under Tsipras, Syriza had governed the country from 2015 to 2019. Recent opinion polls show party support has fallen into single digits.

Associated Press contributed to this report