Three months before Israel’s general elections, all the polls had the Likud becoming the largest party in the next Knesset and Prime Minister Benjamin Netanyahu on a clear course to form the next coalition.

So why did Mr Netanyahu surprise even his closest cabinet colleagues last week by announcing that Likud and Yisrael Beiteinu would be running on a joint candidates’ list?

The deal, which Likud central committee members had little opportunity to scrutinise before being corralled to vote for it on Monday, gives the ruling party no obvious advantage. It safeguards the size of Yisrael Beiteinu’s Knesset faction — the party’s members will have a third of the slots on the list — and guarantees its leader Avigdor Lieberman, the man who wants to supplant Mr Netanyahu as prime minister, the portfolio of his choice in the next cabinet.

So what’s in it for Bibi? The Prime Minister has been worried that his lead in the polls could have a number of adverse effects. The centre-left bloc is currently split between four, perhaps even five different parties, but impending electoral defeat could spur the party leaders into overcoming their differences and uniting. The bloc may even be led by former prime minister, Ehud Olmert, a formidable campaigner who trounced Mr Netanyahu in the 2006 elections.

By joining the two main right-wing parties together, Bibi is staying a step ahead of them.

Mr Netanyahu’s other chief worry is his coalition partners doing too well. Confident that Mr Netanyahu is going to win anyway, religious and right-wing voters may prefer to vote for Shas, led once again by the charismatic ex-convict Aryeh Deri, or the national-religious joint list of Bayit Yehudi and National Union.

The day after the elections, Mr Netanyahu will have to negotiate with these parties, which will be eager for ministries and a say in the new government’s policies. He wants to do so from a position of strength, as head of a much larger party. He is also mindful that Mr Lieberman may be soon indicted for money-laundering and forced to resign, at least temporarily. Leaderless, the Yisrael Beiteinu Knesset members could become loose cannons. This way, Mr Netanyahu has a much better chance of keeping them within the fold.

But Mr Netanyahu and Mr Lieberman’s “October surprise” could end up backfiring on them. Pollsters have had little time to adjust their models and so far there are widely conflicting indications of how well the new Likud Beiteinu will actually do and whether it will gain more or less seats than the two parties currently control.

Also, the alliance may end up alienating two significant groups of Likud voters. The overtly secular identity and policies of Mr Lieberman’s party could push away traditional religious voters, while the moderate Likudniks, who see Mr Lieberman as anti-democratic, may consider a more centrist option.

Not only does the new alliance risk frightening away potential voters, it may just be what was needed to galvanise the centre-left and finally come around to co-operating in an anti-Bibi front.