Israel’s former centre-left prime minister Ehud Barak has given his campaign to unseat the incumbent PM, Benjamin Netanyahu, in upcoming elections a boost by making a pact with leftwing politicians.

The 77-year-old retired from politics in 2013 but returned in dramatic fashion this summer to help end what he said was Netanyahu’s “rule with the radicals, racists and the corrupt”.

Barak, chief of staff of the Israel Defense Forces from 1991 to 1995, once commanded Netanyahu but later served as his defence minister. He has been a vocal critic of the prime minister, who faces three major corruption scandals and has been slammed for allying with far-right political figures.

On Thursday, Barak, another prominent Israeli politician, Stav Shaffir, and the small anti-occupation Meretz party signed an agreement to run together in the 17 September poll under a single party called the Democratic Camp.

“The campaign for the State of Israel began this morning. The Democratic Camp – all of us, together, will fight and win,” Barak wrote on Twitter, announcing the alliance.

Mergers are common in Israeli politics, giving smaller parties that might not cross an electoral threshold a better chance of entering parliament, the Knesset. Meretz barely made it into the Knesset in the last election, while Barak has been polling poorly since he formed his new party, Democratic Israel, in June.

The centre-left Labour party, which Barak once led and was for decades a dominant force in Israeli politics, has also plummeted in popularity in the face of a rightwing surge. There have been calls from within the Labour party to also join the alliance.

Barak has sought to create a powerful bloc of centre-left parties to challenge rightwing parties led by Netanyahu. At 10th place on the new party’s list of candidates, Barak might not himself make it into the Knesset unless the party receives significant votes. Instead, he hopes the stronger alliance will present a better chance to end Netanyahu’s decade-long reign.

The Meretz lawmaker and former party leader Tamar Zandberg called the merger a “dramatic move to strengthen the left” and a “significant boost to justice and equality as an alternative to the corrupt and messianic right”.

Israel is holding elections just months after an April poll in which rightwing parties came out with a clear lead. Regardless, Netanyahu was unable to form a majority coalition government due to infighting between religious and secular parties.

Rather than give his rivals a chance to forge a government, the prime minister’s Likud party instead pushed to dissolve the Knesset, triggering repeat elections.

It is unclear if centre and leftwing parties will form a large enough bloc to unseat the right’s grip on power. Barak, whose premiership ended after the outbreak of a Palestinian uprising in 2000, is a wildcard in 2019 but may not tip the balance.

His election campaign has been tainted by business links to Jeffrey Epstein, the US financier jailed on sex trafficking charges involving dozens of underage girls. Barak, who in his private life became a consultant for private equity and hedge fund firms, said he had “cut all ties” with Epstein.

Separately, on Tuesday, Barak apologised for the killing of 13 Arab protesters by Israeli police at the start of the second intifada under his watch.

“I take responsibility for everything good and bad that happened during my tenure as prime minister, including the events in October 2000 in which 12 Israeli Arabs and a Palestinian from Gaza were killed,” Barak told Israeli public radio.

“I am once again expressing my regret and apology before the families,” he said.

It was widely seen as a move to woo leftwingers, such as Meretz supporters, and Palestinian citizens of Israel – who make up roughly a fifth of the country’s population.

Anshel Pfeffer, a writer for the left-leaning Haaretz newspaper, said Meretz has gained the most from the merger as its party head, Nitzan Horowitz, will lead the Democratic Camp.

However, Meretz also “risks losing some of the party’s more leftwing voters, who will find it extremely difficult to vote for a slate including the arch-capitalist and warlord Barak,” Pfeffer wrote on Thursday.

While army credentials give Barak gravitas among security-minded voters, the opposition is already filled with former military figures, leading to fears his entry to the race could split the anti-Netanyahu vote. The centrist Blue and White party, which drew even with Likud in the 9 April election, is led by former army chief of staff Benny Gantz and two ex-generals.

On Saturday, Netanyahu became Israel’s longest-serving prime minister, spending more than 13 years in office. He is due to face pre-trial hearings for corruption allegations weeks after the vote.

Israel’s attorney-general has already announced his intention to indict him on fraud and bribery charges. Netanyahu denies all charges and would not be legally required to step down unless he is convicted and all his appeals are rejected. That process could take years.

When he announced his return to politics in June, Barak addressed Netanyahu directly “as your former commander” and said he “can’t continue and hold on to the reins of power”.

He said: “For your own good, and for the good of the state and everything you’ve contributed to it during your life, your time as a political leader is over.”