In John Frankenheimer's original, electrifying 1962 thriller, The Manchurian Candidate, an American soldier is captured by communists during the Korean War, brainwashed and programmed to return to the United States and, years later, to assassinate a presidential candidate.

There is compelling evidence now that John Kerry is a kind of Manchurian Candidate of Democratic politics. It seems entirely possible that at some point in his career, he was seized by a youthful Karl Rove, brainwashed and programmed to kill off, at crucial moments in American history, the Democratic party’s political prospects.

The clues were there all along, if we’d only looked closely enough. His curious combination of self-satisfied superiority and baffled indecisiveness was obviously too contradictory a mental characteristic to be natural. His ponderous oratorical style and studied condescension suggested something artificial had interfered with the firing of the synapses.

But the plot worked brilliantly. In 2004, as the party’s presidential candidate, Mr Kerry contrived to throw away a golden opportunity for a Democratic victory against an increasingly unpopular incumbent fighting an increasingly unpopular war.

Startlingly, this week, with the Democrats on the brink of their first clear victory in congressional elections for 14 years, the Manchurian Candidate seized a rare second chance to assassinate his party. Speaking to a crowd of students in California, Mr Kerry mused on the importance of education: “If you make the most of it, if you study hard and you do your homework, and you make an effort to be smart, you can do well. If you don’t, you get stuck in Iraq.”

Now it is entirely plausible that Mr Kerry did not mean to insult the intelligence and diligence of all who serve in the US military, and that he was merely bungling a rather predictable joke about President Bush’s supposed intellectual shortcomings.

But the ambiguity of the remark, coupled with the Massachusetts senator having a bit of form when it comes to making demeaning remarks about American soldiers, was unfortunate to say the least. Whatever Americans think of the war in Iraq, they harbour a deep respect for their men and women in uniform. The clumsy gaffe clearly required an urgent clarification and an apology.

But the Republicans’ most effective secret weapon since Michael Dukakis put on an ill-fitting helmet in a TV commercial and lost an 18-point lead in the 1988 presidential election was not about to let the Democrats off the hook that lightly.

He initially wheedled, refusing to apologise, and rounded instead on his critics. In the process he managed to drag out a no-win story for the Democrats for an improbable two days of news cycles until eventually forced to issue a proper retraction.

It’s still not clear how effective the latest Kerry intervention will be. The senator himself is not running for office in Tuesday’s elections and so this time, Democrats felt free to walk quickly away from the wreckage of their former leader’s self-destruction.

But the incident has shone a rare spotlight, in this critical midterm election campaign, on the Democratic Party. The reason it received so much attention, and so alarmed Democrats, is that it threatened to undermine the party’s entire strategy for taking control of Congress.

Since the election battle was joined months ago, the Democratic approach has been to keep the attention on the Republican Party. As long as the election is about the unpopular President Bush and his fellow Republicans in congress, voters are inevitably much more likely to vote against them. When a party has dominated, as the Republicans have done for the past six years, the debate focuses on their shortcomings, which are not in short supply.

But elections also require voters to choose between alternatives, and Democrats have been extremely anxious not to talk about what they will do if they win on Tuesday. Other than a few old commitments to raising the minimum wage and re-examining tax cuts, there is no 2006 Democratic equivalent of the “Contract with America” that Republicans brought to Washington when they won control of Congress in 1994.

This is partly because the notoriously fractious Democrats can’t really agree on much. On Iraq, the issue most likely to persuade voters to choose them, Democrat policies cover the entire spectrum of possible choices. Some want US troops out immediately. Others back Mr Bush’s “stay the course” approach. Some of the party’s foreign policy leaders back a plan to partition Iraq into three states. Others have described the idea as “suicidal”.

More generally on national security the party is split deeply. On the one hand a growing and vociferous band of radical progressives wants to reconnect with their inner peaceniks from the 1960s and join hands with European lefties in calling for an end to the abuses of American power. On the other, hawks such as Hillary Clinton have criticised Mr Bush at times for being too soft on Iran and “contracting out” US diplomacy to Europe.

Democrats are also unwilling to show their hand because it may scare the voters. They have moved sharply to the left since Bill Clinton’s new Democrats won in the 1990s. Anger and resentment at Mr Bush’s brand of conservatism have curdled inside their party to a point where many of its activists no longer care about reaching the middle ground.

The party’s leaders, such as Nancy Pelosi, who will become Speaker of the House is among the most left-wing of House Democrats. On economics, the party has abandoned Clintonian pragmatism for naked populism.

The glimpse of Democratic leadership afforded by Mr Kerry’s intervention probably came too late to deprive the party of a majority in at least one and possibly both Houses of Congress next week. But as Democrats prepare the celebrations most of them know that their problems are just beginning.

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