For the first time in six years, Prime Minister Benjamin Netanyahu's premiership seems to be under serious threat.

The latest polls have consistently shown the Zionist Union list, headed by Labour Party leader Isaac Herzog, a few seats ahead of Mr Netanyahu's Likud.

As things stand now, on the morning after election day, next Wednesday, both party leaders will have a tough time forming a coalition.

The last month of campaigning has seen a steady erosion in Likud's strength, caused partly by a series of revelations and scandals regarding Mr Netanyahu's wife and alleged public funding of his household, but in the main by the successful focus of his rivals on cost-of-living issues, particularly the housing crisis. In the polls last weekend, it seemed that Mr Netanyahu had succeeded in stemming the decline and, in the wake of his speech to the US Congress on the Iranian threat, Likud once again closed the gap with Zionist Union. But by midweek, polls had the party back down to only 21 Knesset seats, trailing Zionist Union by three or four seats.

While this is good news for Mr Herzog, there are still major obstacles facing him. None of the polls gave the four parties that are resolutely opposed to another Netanyahu government - Zionist Union, Meretz, Yesh Atid and the Arab Joint List - a blocking majority. Together, they have at most 56 seats, still leaving the possibility of Mr Netanyahu forming a coalition with the right, the religious parties, and the social-affairs orientated party led by former Likud minister Moshe Kahlon.

In nearly every scenario, Mr Kahlon is the kingmaker who holds the keys to the prime minister's office. He remains opaque as to his preference and will stay that way until after the votes are in. One Likud official who knows him well said that, "Kahlon will have to decide what is stronger. His dislike of Netanyahu, or his hope to come back to the Likud and lead the party one day. If he helps Herzog form a government, the party will never forgive him."

Mr Herzog's greatest headache, however, is Yesh Atid, the secularist-centrist party led by Yair Lapid, which is likely to be the third largest party in the next Knesset with 12-14 seats.

In order to form a coalition, Mr Herzog would need Mr Lapid's seats but also those of the Charedi parties, who have sworn not to sit in a cabinet with Yesh Atid.

On Tuesday, Shas leader Arye Deri said that he was open to joining a Herzog government. "I said I would recommend Netanyahu but if the president gives the mandate to Herzog, I certainly am not ruling him out." But regarding Yesh Atid, he said the chance of Shas joining a government with them was "close to zero".

Ultimately, the dilemma could fall to President Reuven Rivlin as both Mr Netanyahu and Mr Herzog are likely to announce victory as the results arrive in the early hours of Wednesday morning. Mr Rivlin has already said that he will not necessarily give the mandate to form a government to the leader of the largest party, but to the candidate with the best chance of commanding a majority in the new Knesset. In 2009, Likud received one seat less than Tzipi Livni's Kadima but Mr Netanyahu went on to form the government.

However, even if the right-wing-religious bloc turns out to be larger on paper, Mr Netanyahu will have trouble claiming a mandate from the public for a fourth term if the gap is four seats or wider.

Mr Netanyahu is fully aware that this is a battle for his political future. To close the gap with Zionist Union, he is trying to attract right-wing voters away from Likud's "satellite" parties, Jewish Home and Shas.

"It is a very close fight, nothing is assured," he told party activists on Monday. He added: "There is a massive effort, crossing the globe, to topple the Likud." He has three more days to convince Israelis that this threat is real and he is the man to stand up to it.