What went wrong? This week two of the most successful centre-left politicians of their generation are asking themselves this question as their dreams of glory collapse. While Hillary Clinton at least has the consolation of carrying on her political career as a respected and powerful senator, perhaps even as vice-president in a Democratic “dream ticket”, Gordon Brown can only look forward to two years of parliamentary humiliation, internecine backstabbing and lame-duck impotence, followed by electoral defeat. At the moment, however, the common features of these two defeated politicians are more interesting than the many differences between their plights.

What Mr Brown and Mrs Clinton most obviously have in common is their personality, or lack thereof. Both are quintessential machine politicians, obsessed with tactical calculation and policy detail; lacking in eloquence, likeability and unifying vision. These personal deficiencies suggest an obvious conclusion: that the salvation of progressive politics on both sides of the Atlantic will depend on the emergence of charismatic new leaders, Barack Obama in America and a still-undiscovered reincarnation of Tony Blair over here.

There are, however, three other plausible explanations for the Clinton-Brown debacles with much deeper implications for progressive parties everywhere than the simple injunction that they must choose a leader with a charming smile and a good turn of phrase.

The first is Iraq. This is the elephant in the room that has been inexplicably ignored in much of the commentary on where Clinton and Brown went wrong. If Mrs Clinton had not backed President Bush at the start of the Iraq war and for most of the period since, the challenge to her candidacy would never have got off the ground and Barack Obama would still be an obscure local politician from Illinois. If Mr Brown had broken decisively with the Blair-Bush foreign policy that he inherited, as many of his supporters (including me) hoped, then the disillusionment with his leadership, among both traditional Labour activists and the champagne-socialist intelligentsia, might never have set in.

Moreover, a clear shift of policy in this one vital area would have created the genuine sense of change and post-Blair political renewal. This would have obviated ludicrous expedients such as the renaming of government departments and plagiarism of Tory tax policies with which Mr Brown tried to create the illusion of change but succeeded only in drawing attention to his lack of ideas.

Worse still, Mr Brown's “have your cake and eat it” approach to Iraq revealed a second political characteristic he shared with Mrs Clinton, which has been much more damaging politically than lack of ideas. This was their almost addictive use of “triangulation”, the fancy political name for what might otherwise be called hypocrisy, saying one thing to one audience and the opposite to another, in the hope of winning support from both sides: for the war in Iraq and against it; for lower taxes and more redistribution; more public services and tighter control of government spending; less regulation and more state planning; greater individual freedom and more encroachment on civil rights; “evidenced-based policymaking” and media-driven knee-jerk actions such as yesterday's U-turn on drugs laws.

Such policy contradictions are not in themselves politically disastrous; in fact they exist in every party manifesto and government programme.When politicians are doing well, triangulation works in their favour: audiences trust in their integrity and share their vision. They want to believe what they are told, even if these words are contradicted by other speeches or actions.

This is why Bill Clinton and Tony Blair were masters of triangulation (indeed, they introduced the word to the political vocabulary) and why Mr Obama seems to have survived the controversy over his relationship with the Rev Jeremiah Wright.

But when politicians' reputations are sliding, this dynamic goes into reverse. Instead of getting credit for both the positions they espouse - for example, better public services and lower taxes - the public gives them credit for neither.

Such a loss of credibility is particularly dangerous for left-of-centre politicians because of the last, and probably the most important, common feature of the political reverses suffered by Mrs Clinton and Mr Brown.

This is the alienation of prosperous, idealistic middle-class voters and the resulting loss of support from the elite liberal media that tend to reflect their views. For right-wing parties the attitudes of the bien-pensant liberal media do not matter much, because conservative politicians who advocate low taxes and small government can generally appeal to most prosperous voters on the basis of pure economic self-interest. Left-of-centre parties, by contrast, can only achieve power by creating a coalition of economically motivated working-class voters and highly educated middle-class voters, who vote for left-wing parties because of their liberal ideals.

That, at least, seems to be the lesson of recent history, especially since the end of the Cold War. A left-wing politician who loses the support of this liberal constituency is probably doomed.

This happened to Mrs Clinton when idealistic middle-class voters and their favoured media outlets decided that she had lost her integrity over Iraq - and also through her alleged racist insinuations against Mr Obama, which were for some reason considered more despicable than the many patronising and demeaning insinuations about Mrs Clinton's femininity.

In Britain, Mr Brown has been undone by a similar process of middle-class disillusionment. This started with his prevarication over Iraq and the way he insulted the intelligence of the voters last autumn, starting with the election that never was, continuing with the plagiarism of Tory tax plans and climaxing with the dithering incompetence of the Northern Rock crisis.

He might still have kept some support among the liberal intelligentsia had he not embarked on an extraordinary campaign of illiberal measures this year - his campaign for 42-day detention without trial, his insistence on identity cards, his attacks on church schools, his veto against a pay rise for prisoners and now his irrational policy reversal on cannabis, which must make it almost impossible for any liberal-minded voter to support Labour. With all of these policies, Mr Brown appears to be pandering to the editorial columns of the Mail and the Telegraph, instead of The Guardian and The Independent.

If Mr Brown thinks he can create a winning coalition for Labour from readers of the Mail and Telegraph, he will indeed turn out to be the greatest left-of-centre politician of his generation. Failing that, the future of progressive politics now rests with Barack Obama.

Times Recommends

• It’s not just about Iraq, it’s about Blair’s style
• You can’t rank hospitals like football teams
• Minarets are not an essential part of Islam