By some yardsticks, Japan's electorate has delivered the biggest shock to the political system since the end of the American occupation. Voters have shattered, at a stroke, what came to be known as the “1955 system” – five decades of political stability administered by a Liberal Democratic party whose conservative, but egalitarian, instincts have suited the postwar temperament.

First through a period of transformational growth and, latterly through years in which the economy has been treading water, the LDP has time and again been entrusted with the keys of office.

Now Japan's electorate has changed the locks. Despite the drizzle, voters yesterday flocked to polling stations to register their dissatisfaction with politics as usual and their willingness to gamble on something new.

As LDP heavyweights, some of whom lost their seats, seek to work out what has hit them, and as the untested Democratic party adjusts to power, two interrelated questions need answering. First, what has brought about this seemingly dramatic break with the past? And, second, what precisely has Japan's electorate voted for?

There is no doubting the political symbolism of this crushing defeat. Electors, who for decades have had no real choice, have seized the opportunity to end one-party rule. Their revolt comes after years of policy drift that have exposed the weakness of an LDP that knew how to bring about economic parity with the west but was unsure what to do next.

The party has struggled to come to grips with the aftermath of the 1990 bursting of the bubble, which left Japan in seemingly terminal decline relative to other countries, particularly its resurgent neighbour China.

Even more than the party's failure to deal with a difficult present, the electorate has worried about its inability to summon convincing ideas about how to confront a future in which the labour force seems destined to shrink, the population age and national debts mount.

Yet the shifting of political tectonic plates – “small earthquake, no tsunami”, as one astute observer put it – is not quite as tumultuous as meets the eye. The result did not come out of the blue. Rather, it echoed two previous general elections that, in retrospect at least, marked the slow-motion crumbling of the LDP edifice.

Its first intimation of death came as long ago as 1993. Then the LDP lost power fleetingly – and almost accidentally – to an eight-party coalition. That election was not a decisive vote against the LDP, which remained the biggest single party by far.

Rather, it reflected the fragmentation of a party whose argumentative factions had for four decades found a common cause in “GNPism” and cold war certainties. When the bubble burst and the Berlin Wall fell, the LDP stuttered.

The second decisive stake in the LDP's heart came in 2005, paradoxically an election it considered a triumph. Four years ago, under Junichiro Koizumi, it won a two-thirds majority. But Mr Koizumi was running on an “anti-LDP” ticket, threatening to smash his own party if it did not bend to his free-market agenda.

Voters, fumbling for a way to break with the past, took up Mr Koizumi's challenge. It was a dress rehearsal for last night's punishing verdict.

What does that verdict mean? More than a vote for the DPJ, an amalgam of five disparate parties with widely different ideologies, the electorate has instead voted against the tired and discredited incumbents. Rather than offering radically new policies, the DPJ's emphasis on inclusive capitalism, via income support and regulation, often sounds like a throwback to the LDP of old.

That is because it has consciously sought to appeal to Japanese who are still fairly content with their lot, in spite of concerns over widening income disparities and the threats of untamed globalisation. More than disquiet about the present, voters are nervous about the future. They worry about their pensions and healthcare, and the job security of their children and grandchildren. Viewed in that light, yesterday's vote for change looks more like an attempt to preserve the status quo by other means.

Yesterday's upset was the culmination of a long-drawn out rejection of the postwar political order. It has been delivered without real conviction and without the intention of catalysing a social revolution. It is a very Japanese revolt, but a revolt nonetheless.