"We do need a new politics in this country," declared the leader of the opposition. "Ordinary people have become unprecedentedly cynical about politics and politicians. There is only one way out of this national crisis that we face. Transform our politics ... give power to the people."

I've cheated you a little there. That is not one, but two leaders of the opposition speaking. The first and third sentences come from David Cameron's recent speech in which he presented himself as the people's champion, the blue Robespierre of Notting Hill. The second and fourth sentences belong to a speech made in 1996 by Tony Blair when he was offering himself as the pink Danton of Islington. When he arrived in office a year later, Mr Blair demonstrated little enthusiasm for sharing power with most of his cabinet, never mind the people. Always beware any leader of the opposition who says he only wants to seize power in order to give it away.

David Cameron was not saying much at all about democratic reform before the expenses scandal exploded on Westminster: a dirty bomb that continues to spread toxic fall-out on both sides of the aisle. Deflection is one reason why both the Tory leader and Gordon Brown have seized on the subject of the constitution. A debate about that may, or so they rather vainly hope, lift the nation's gaze from all the squalid scams involving champagne flutes, swimming pools and phantom mortgages.

The next reason is positioning. The Tory leader has long been trying to seduce progressives with the idea that he is a "liberal Conservative" who is as appalled as they are with the control-freakery of the Labour years. The expenses scandal provides another opportunity to whisper some sweet nothings about reform. The Tory seducer hopes to weaken the knees of liberal Britain with the romance of a "massive, sweeping, radical redistribution of power". What he's most ambitious to achieve is a massive redistribution of power from Gordon Brown to David Cameron. By their works shall ye know them. The highly centralised style of the leadership of the Conservative party does not suggest that he is genuinely one of nature's pluralists. As for transparency and accountability, we still do not know whether Michael Ashcroft, the deputy chairman and Tory sugar daddy, is a British resident for tax purposes.

The segments of the speech with most promise were those which relied for their ideas on Vernon Bogdanor, the professor of politics who taught PPE to David Cameron at Oxford. In his latest book, The New British Constitution, Professor Bogdanor makes an insightful observation about why devolution and Labour's other constitutional reforms have not rejuvenated politics. They redistributed power not between government and people, but between elites, between politicians and judges, and between one set of politicians in London and others in Edinburgh, Cardiff and Belfast. That persuasive critique provided the basis for the most compelling section of Mr Cameron's speech. It would indeed be empowering, though also with considerable complications over which he skated, if the public truly gained the capacity to hold to account the people who run their police forces, hospitals and schools.

He is on more familiar, though very swampy, ground when he demands a repatriation of power "from Brussels to Britain". About this, the Conservatives are both deadly serious and dangerously flippant. We have yet to hear from the Conservative leader how he thinks he will convince the other 26 members of the European Union to renegotiate the complex power relationships within the EU. "In so far as there is a plan, it is in William Hague's head," says one senior Tory, unconvincingly. It is not at all obvious why Paris, Berlin, Madrid, Rome and the rest would co-operate with a renegotiation at the behest of a Conservative party which is so short of friends in Europe that they are allying themselves with the ultra-reactionary Polish Law and Justice party and the crazily right-wing Fatherland and Freedom party of Latvia.

David Cameron is timid when it comes to reform in Britain. He sees the objection to privileging whoever is prime minister with the ability to try to fix the race by calling an election at any time of his or her choosing. He says he will "seriously consider" introducing the fixed-term parliament. Yes, I am sure he would think about that for all of a sub-nanosecond after he had stepped into Number 10. The power is not always such a boon: Gordon Brown used the starting gun to blast a huge hole in his own foot when he dithered and swithered about calling an election in autumn 2007. Harold Wilson, Ted Heath and Jim Callaghan all mistimed elections. But no prime minister has ever seriously considered giving up that power once he had his sticky hands on it. I will not believe that David Cameron has any intention of delivering unless the promise is written in his blood in his manifesto and then I will continue to doubt it until the legislation to introduce fixed-term parliaments has received the Royal Assent.

Electoral reform of the Commons is the big one. The Conservatives remain viscerally opposed to changing a voting system which effectively creates two kinds of rotten borough. In the large number of constituencies where the contest between the parties is never competitive, getting selected as the Conservative or Labour candidate means being given a seat for life - bar disgrace.

One of the origins of the expenses scandal is the complacency this can breed in MPs sitting on seats that are forever safe. In a smaller number of seats, the 100 to 150 marginals where swing voters determine the outcome, MPs are sent to Parliament never having secured the positive endorsement of half their voters. In the last election, not a single MP sent to the Commons had the expressed support of more than 50% of their total electorate.

The Tories have got the rough end of first past the post at the last three elections, because it exaggerates the penalty for being the runner-up as it also amplifies the reward for coming first. In 2005, it was worse than that. The Tories won 60,000 more votes in England than Labour, but Labour took 92 more seats. Labour is profoundly unpopular now. But then Labour has not been popular for a long time. They won a parliamentary majority of 66 with the endorsement of barely over 1 in 5 of the electorate.

The Tories nevertheless remain attached to first past the post, not least because they expect to be the big beneficiaries of winner takes all at the next election as they were for most of the elections of the 20th century. One promise that I do believe from the Tories is that they will implement a sweeping redistribution of constituency boundaries to make more equal the number of voters in each seat. This will not make each vote equal in value. It will have the effect of inflating the number of Tory MPs and culling the ranks of their opponents. After a decade in which the system has been tilted against the Conservatives, they are going to make jolly sure that in future the bias is in their favour.

As Labour contemplates losing power, some of its senior figures have suddenly remembered that they are mustard-keen on voting reform. One of them is Alan Johnson, the cabinet minister thought most likely by his colleagues to succeed should Gordon Brown fall - or be pushed - under a bus. Roy Hattersley, with whom I had many good-natured arguments during his years as a supporter of first past the post, is now a convert to reform.

It is all far too late for Labour. You cannot step in the same river twice. The time for Labour to be serious about electoral reform was after the 1997 landslide when it could have changed the voting system from a position of parliamentary strength and moral authority.

There were several reasons why Tony Blair reneged on the promise to hold a referendum, left Paddy Ashdown jilted at the altar of coalition and betrayed his favourite uncle, Roy Jenkins. Of those many reasons, one of the most important was the implacable opposition of Jack Straw, David Blunkett, John Prescott and, most of all, Gordon Brown. As they sowed, now shall they reap.

A minority of Labour politicians are sincere on this subject, but Labour as a party is completely untrustworthy on electoral reform. It flirts with the idea when it is in opposition or becomes afraid that it is about to lose office. As soon as it has power, Labour forgets its promises.

Only one of the main parties has been a consistent and persistent advocate of a democratised House of Lords, a reformed Commons, fixed-term parliaments and the rest of the menu of truly sweeping constitutional reform. That party is the Liberal Democrats. They have also, incidentally, come out of the expenses scandal much less sleazed than either Labour or the Tories.

Of course, it is self-serving of the Lib Dems to want voting reform. A more proportional system would almost certainly give Nick Clegg a larger - that is to say, fairer - number of MPs. Precisely because they stand to gain, we can rely on the Lib Dems to remain committed to reform long after the expenses scandal has faded from the headlines.

This is not an endorsement of the Lib Dems. It is simply an observation, which perhaps makes the point more powerfully. If you want to maximise the chances of securing serious constitutional reform, then the party to vote for is the Lib Dems.