According to all conventional instruments of political analysis the Japanese prime minister, Taro Aso, is utterly doomed. His ratings are not much above 20 per cent, his Liberal Democratic Party has just suffered a humiliating defeat in the Tokyo local elections, and plenty of his own allies are demanding that he quit.

Yesterday he called a general election which the LDP will almost certainly lose – for only the second time since the Second World War – because there were no better options available to him. His own humiliation as a politician is perhaps the best he can hope for – it is quite possible that he will preside over the break up and disintegration of one of the world’s most successful political parties.

And yet something makes one wary of writing the obituary of the LDP. It certainly faces a political roasting, and it is hard to see how it can avoid losing power in the August 30 election to the opposition Democratic Party of Japan (DPJ). But the LDP will not go without a fight, and it is still possible to imagine a miraculous return from the abyss – if not before the election, then a few months afterwards.

Several things have ensured the LDP’s survival over the past five and half decades, and may yet save it in the future. Until now it has not been bettered as a vote-getting political machine at the local level – the level at which all elections are won and lost. Local grass roots organisation is only one of several ways in which the DPJ has consistently proved deficient.

In the decade since its foundation, out of a rabble of former socialists, and left and right-wing defectors from the LDP, the Democrats have struggled to create a united identity, and have failed to take advantage of the many missteps and weaknesses of successive governments. It is not enough for the LDP to lose this election – the DPJ must also win it, and as a party it has a long-established habit of shooting itself in the foot at crucial moments.

In May, it lost its then leader, Ichiro Ozawa, after prosecutors produced evidence that his office had received improper political donations. A similar, though milder, funding scandal is now smouldering around his successor, Yukio Hatoyama. The timing of these investigations has been so convenient from the government’s point of view that many Japanese speculate about politically motivation – whatever the truth, the DPJ seems to be more susceptible to the blast of such political landmines than the LDP.

But the LDP has a far more potent advantage: the profound reluctance of most Japanese people to vote for anyone else. There is a reason why a two party system of alternating parties has never taken root in Japan, and why the party has been elected and re-elected, despite 15 years of slump and recession: the electorate keep choosing them. It sometimes seems as if the LDP is part of the DNA of Japanese people.

The LDP has lost only once before, in 1993, when Mr Ozawa himself deserted the party with a group of rebels who succeeded in replacing the Government with a new coalition. At the time, as now, the change was described as a revolution, which would change Japanese politics forever. After fewer than 11 months, a confused and chaotic period, the LDP was back in power. They have been in charge for so long, it is impossible to imagine anyone doing the job. Voting them out of power would feel like democratically ousting your Mum and Dad.

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