To judge by Barack Obama's disappointing performance so far in the opinion polls, reflected in the surprisingly subdued atmosphere at the Denver convention, Democrats are suffering a bad case of “buyer's remorse”.

This distressing psychological syndrome, precipitated by the purchase of a superficially attractive, but unaffordably expensive or inadequately researched consumer item, could only have been intensified by the impressive performances of Hillary and Bill Clinton, which dominated all other events in Denver this week.

It is still possible, of course, that a virtuoso rhetorical display tonight by Senator Obama will galvanise not only his 70,000 adoring fans at the Denver Invesco Stadium, but the US electorate. It is just as likely, however, that any displays of mass hysteria tonight will put off large numbers of voters and feed Republican mockery about “Obamamania”.

If this week's convention fails to achieve the widely predicted take-off in Mr Obama's ratings, does this mean that the presidential election is all but over before it started and that the world must prepare for another four years of Republican rule? The answer is an emphatic “no”.

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The Democrats' Byzantine nominating procedures and their introverted ideas about electoral “fairness” have led them to choose the less electable of their two main candidates. As a result, they have blown the chance of turning the disaster of the Bush presidency into a Roosevelt-style electoral landslide. It is tempting to conclude that the Democrats have again thrown away an easily winnable election. Tempting, but not yet right.

It is too early to write off the Democrats, despite the poor start to their campaigning, because as polling day approaches, voters will realise that this election is not about the Democrats or Mr Obama or his relationship with the Clintons. It is about the Republicans and John McCain and his relationship with George Bush.

The maxim that “oppositions do not win elections; governments lose them” is not just a journalistic cliché. It is a profound statement about democracy. Since nobody can predict the future, it is impossible for voters to base their judgments on whether a future government will be successful or an untested candidate will make a good president.

Manifestos are rarely worth the paper they are written on. This is not just because politicians are dishonest but because unexpected events intervene. Dealing with the unexpected is a much more important function of government than implementing manifestos. Some politicians who seem well prepared and have clear policy objectives, such as Gordon Brown or Richard Nixon, turn out to be hopeless leaders, while others with little experience and few policy positions, such as Ronald Reagan or Tony Blair, end up being successful. Democracy is largely a gamble about who might govern best; and the right to vote is little more than a right to roll the dice.

Why then, do millions of people the world over fight for this right? Because the most important function of democracy is not to choose good governments but to throw out bad ones. It is the right to eject bad governments that prevents tyranny, makes government serve the people, discourages corruption and keeps most democratic nations at peace most of the time.

The corollary of this observation is that politicians must always live in fear of punishment by the voters. But if voters repeatedly fail to punish incompetence or corruption or gross misjudgment, then the fear of defeat is lifted and democracy loses its disciplining power. And a country in which the dominant parties can afford to scoff at the discipline of the ballot box, is the point when democracy starts to slide into self-perpetuating oligarchy.

If the Republicans can get their candidate re-elected to the White House after all their failures of the past eight years - after the military misadventures, the geopolitical blunders, the economic mishaps and the mismanagement of natural disasters - America will be perilously close to the point when democracy ceases to perform its most essential function of disciplining political power.

It may be objected, of course, that the incompetence and misdeeds of George W. Bush should have no bearing on whether John McCain should become president. This, indeed, seems to be the basis of Senator McCain's strategy, which has emphasised his disagreements with President Bush. But even if it were not for the many similarities between the Bush and McCain platforms - aggressive militarism, contempt for international opinion, social conservatism, tax cuts for the richest voters, dogmatic faith in market forces even when, as in energy or housing, they have obviously failed - a Republican win in November would be an affront to American democracy for a deeper reason.

Whether or not Mr McCain would continue the policies of President Bush (and much of the evidence suggests that his would be a Bush presidency on steroids), he would keep in power the coalition of interests that the Republican Party represents: the energy and military-industrial lobbies, the religious conservatives, the anti-environment interests and the neoconservative think-tanks. These groups - which have gained enormous influence, both financially and intellectually, under President Bush - are as responsible for the blunders of the Bush Administration as Mr Bush himself, arguably more so, given the President's notorious lack of interest in the details of any of his own policies.

If a Republican is again elected president, these same centres of power will continue to dominate Washington. However many wars they encouraged, however high the price of oil rose, however many tax dollars were redistributed in their favour, the neoconservatives and Pentagon contractors and religious fundamentalists and oil and Wall Street lobbies would conclude that there would be no political price to pay for failure. They would be justified in concluding that there is no longer any democratic check on their ambitions.

It is only by ejecting the Republicans from the White House that American voters can send the message that they are still in charge of their country and that gross government incompetence will not go unpunished. Accountability - not personality or rhetoric or colour or age or gender - should be the overriding issue in this election. The Democrats - with their naively high-minded focus on Mr Obama's alleged achievements instead of the Bush Administration's manifest blunders - do not yet seem to have understood this. But with luck, American voters will prove less naive than the Democratic high command.

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