Israeli politicians battling to unseat Prime Minister Benjamin Netanyahu have until Wednesday midnight to make their "change" coalition, composed of bitter ideological rivals, viable.
The chances looked auspicious late Wednesday when it emerged Mansour Abbas, head of the Islamic conservative Raam party, had signed the agreement.
They only have until the end of the day – midnight local time – to put together an administration that would end 12 straight years of rule by the hawkish heavyweight, Israel's longest-ruling prime minister.
The push is led by former TV presenter Yair Lapid, a secular centrist, who on Sunday won the crucial support of hardline religious nationalist Naftali Bennett, a tech multi-millionaire.
"The coalition negotiation team sat all night and made progress towards creating a unity government," a spokesman for Mr Bennett said.
But to reach a 61-seat majority in the 120-seat Knesset, their unlikely alliance would also have to include other left and right-wing parties – and would probably need the support of Arab-Israeli politicians.
That would result in a government riven by deep ideological differences on flashpoint issues such as Jewish settlements in the Israel-occupied West Bank and the role of religion in politics.
Mr Lapid, leader of the Yesh Atid party, was asked to form a government by President Reuven Rivlin after Mr Netanyahu again failed to put together his own coalition following Israel's fourth inconclusive election in less than two years.
Mr Lapid has reportedly agreed to allow Mr Bennett, who heads the Yamina party, to serve first as a rotating prime minister in a power-sharing agreement, before swapping with him halfway through their term.
On Tuesday, a source close to the negotiations told AFP that negotiators were hammering away to "finalise a deal as soon as possible".
Benjamin Netanyahu's grip on power loosens
Israel's latest political turmoil adds to the woes of Mr Netanyahu, who is on trial for criminal charges of fraud, bribery and breach of trust while in office – accusations he denies.
If he were to lose power, he would not be able to push through changes to basic laws that could give him immunity, and would lose control over certain Justice Ministry nominations.
Mr Netanyahu, who served an earlier three-year term in the 1990s, had warned fellow citizens on Sunday of "a left-wing government dangerous to the state of Israel".
The 71-year-old, who heads the right-wing Likud party and has developed a reputation as a wily political operator, was scrambling to scupper the new alliance.
Likud's lawyers tried to hobble the emerging coalition by challenging Mr Bennett's right to serve first as prime minister, given that it was Lapid who was invited to form the government.
But the legal adviser to Israel's president knocked down the challenge.
Opponents of the possible alternative government, meanwhile, accused Mr Bennett and his right-wing partners of betraying their voters.
Spokesmen for Mr Lapid and Mr Bennett confirmed to AFP that the two have received additional security protection.
To build the anti-Netanyahu bloc, Mr Lapid must sign exclusive agreements with seven parties, whose members would then vote in parliament to confirm their coalition.
They include the hawkish New Hope party of Mr Netanyahu's former ally Gideon Saar and right-wing secular nationalist Avigdor Lieberman's pro-settlement Yisrael Beitenu party.
The centrist Blue and White party of Defence Minister Benny Gantz, the historically powerful centre-left Labor party and the dovish Meretz party would also join.
Arab Israeli support?
If all those parties indeed sign on, the emerging alliance still needs the backing of four more legislators.
For that, Mr Lapid is counting on parties representing Palestinian citizens of Israel, and late Wednesday he secured the backing of Mansour Abbas, head of the Islamic conservative Raam party, which has four seats.
Mr Abbas had voiced openness to any arrangement that improves living conditions for Israel's 20 per cent Arab minority of Palestinian descent.
Political analyst Afif Abu Much said on Tuesday that Mr Abbas would not pursue ministerial posts, but wanted chairmanship of two parliament committees and budgets for Arab communities.
He also aimed to revoke a law that has hardened penalties for illegal construction, which is seen to affect Arab communities disproportionately.
"They don't want to be part of the government," Mr Abu Much told AFP. "What they want is to be the address of the Arab people in Israel."
Political scientist Jonathan Rynhold said it would be unwise at this point to write off Mr Netanyahu, "the best card player by miles".
If Mr Lapid fails to gain a majority, and MPs cannot agree on another candidate for prime minister, Israelis will return, yet again, to the polls.
Mr Abbas told reporters on Tuesday that negotiations appeared to be heading "in a good direction".
But, he said: "until it's finished, nothing is finished".