The policy innovation added a frisson to the atmosphere around Leinster House last week with many arguing that the positive reaction to the move was evidence that the upcoming general election is far from a done deal.

It would be nice to think that that is the case. It would make the election itself far more interesting but also, a healthy democracy requires the electorate being presented with a viable alternative at each election. Last week marked the 20th anniversary of Fianna FA il's 1987 general election success and it has been in government virtually ever since. As this column has regularly pointed out, FFs decision to abandon its objection to coalition government has resulted in Ireland becoming the equivalent of a one-party (or more accurately a 1.1 or 1.2 party) state.

But despite growing optimism that the Rainbow parties can now make a real contest of the general election, a cold-eyed look at the figures clearly demonstrates the odds are stacked against Fine Gael, Labour and the Greens having the numbers to put a government together come next May.

To succeed, the three parties will need to win a minimum of 80 seats between them - enough to form a government with Independents' support.

The problem is that the three parties have a combined total of just 59 seats currently and need a net gain of a minimum of 21 seats to be in any position to form a coalition. No opposition in the history of the state has overcome a gap that size to win power.

They will certainly make gains. The Greens, for example, are shaping up to win as many as 10 seats in the election, a gain of four seats. But even if that comes to pass, it still leaves FG and Labour needing to win 70 seats between them. Labour looks to be in good nick right now, but given that at least four of its current seats look very marginal, it would be a remarkable feat if it returns with 25 TDs after the election - a gain of four. That leaves FG needing to win 45 seats - up 14 from its 2002 meltdown - to complete the Rainbow.

Each of the three parties individually is capable of hitting that target. But bearing in mind the inevitability the three parties will take seats from each other, it is asking a lot for all three to do so simultaneously.

Their task looks even more daunting if we look at the other side of the equation. If FG/Labour is to form a minority government, it has to restrict the number of non-Rainbow TDs - Fianna FA il, the PDs, the Socialist Party and Independents - to an absolute maximum of 86 seats.

Fianna FA il has the luxury of starting each general election with effectively 43 seats in the bag as history shows us the party always wins a seat in every constituency. That could change in May, but there are only two constituencies - Kerry North and Dublin South-East - where there is even a remote possibility of that happening. Even if we assume FF goes into meltdown and has its worst-ever election result, there are also a minimum of 12 constituencies where it is effectively guaranteed to win a second seat (Laois-Offaly and Cork South-Central to name but two).

To that total of 55 seats can be added a further 10 constituencies where it is extremely difficult to imagine Fianna FA il not winning a second seat (including Dublin South, Cork North-Central, Clare and Longford/Westmeath). That brings the party to 65 seats - three lower than it got in its 1992 disaster under Albert Reynolds. But even this figure assumes the party will lose its second seat in all five northside Dublin constituencies, along with Dun Laoghaire, Dublin South West, Dublin South Central, Limerick West, Mayo, Louth and Donegal South-West and fails to pick up a second seat in Meath West, Roscommon-Leitrim South or SligoLeitrim North - 15 constituencies where the party has at least a 50-50 chance of securing a second seat.

The brutal reality is that for the Rainbow to be able to form even a minority government, it needs Fianna FA il, the PDs, Sinn FAcopyrightin and 'Others' to all have a disastrous day at the polls. It needs all four at close to their absolute floor level of seats. One can never say 'never' in politics but - particularly when recent polls are factored in - it looks a massive ask.

In that unlikely scenario, the PDs could drop to two seats (two below its 1997 disaster), Sinn FAcopyrightin could dip to four, while 'Others' might halve to seven. But even that worst-case scenario yields as many as 78 non-Rainbow TDs - only eight shy of the number needed to deny the Rainbow.