I don't blame Shirley Williams, though lots of people do. There are some who have always loved blaming Shirley Williams, for everything from the abolition of grammar schools to the 18 years that Labour spent out of power - and probably last summer's bad weather too. The Lib Dem peer's sin this time, apparently, was to hold Sir Menzies Campbell to ransom by threatening to defect back to Labour unless he dropped his manifesto commitment to hold a referendum on the Lisbon treaty.

This story is disputed by people close to Sir Ming, which is why I will hold the baroness innocent until proved guilty. But it is certainly the case that a whole group of European integrationists took advantage of the elderly Liberal Democrat leader's weakness last autumn to make him change the party's position on a European referendum in the run-up to what could otherwise have been a tricky party conference. Instead of supporting a vote on the Lisbon treaty - the successor to the failed EU constitution - the Lib Dems moved to calling for an “in or out” referendum on EU membership.

Or at least some of them did. Tonight, in a Commons vote on an amendment calling for a referendum on the Lisbon treaty, ten or fifteen Lib Dem MPs will defy their party's three-line whip and support the amendment. The rest will abstain, as instructed. The latter's action - or rather inaction - will make a mockery of the new leader's professed desire for a new politics. By supporting an “in or out” referendum, Nick Clegg thinks he can pretend to be in favour of the people having a say without any danger of imperilling the Lisbon treaty. Like Hillary Clinton, like Gordon Brown, Mr Clegg will strike voters as the classic opportunistic politician, ducking and weaving as it suits him politically, happy to break promises if they prove inconvenient.

It won't wash. For Obamamania is spreading to these shores too. There is a longing for a new style of politician who does not feed public cynicism, but challenges it. Voters don't want to be taken for fools; they demand to be treated as intelligent beings. To be told that we don't need a referendum on the Lisbon treaty because it is so different from the constitution, when leaders in Europe have been lining up to tell us that the two documents are almost exactly the same, is an insult to our intelligence. Mr Clegg should know better.

It will damage his party, just as it is damaging Labour. The past few political scandals have infected all three parties: dodgy peerages, dodgy party funding, dodgy expenses for MPs. This reneging on a manifesto commitment, though, which will fuel public mistrust just as powerfully, affects only two parties out of the three. For David Cameron's Conservatives are still supporting the promised referendum. No wonder the Tory leader basked in a little Obama fairy-dust at the weekend by borrowing the American candidate's “broken politics” line and applying it to Westminster.

Bizarrely, given their recent history, the Conservatives have been the most united of the three parties in this European treaty debate. Only a handful of Europhile stalwarts - Ken Clarke and three or four allies - have voted against the party line. By contrast, the Lib Dems are deeply split and Labour has 30-odd rebels, despite some heavy-handed work by the whips. Geoff Hoon, the Chief Whip, summoned Gisela Stuart to a private meeting and screamed at her. Kate Hoey and Frank Field received similar treatment for their support for the I Want a Referendum group, which has been holding private referendums in Labour and Lib Dem marginals.

“Why don't you hold one in my [safe] seat instead?” spat out Mr Hoon. “I can fix that too, if you like,” replied Mr Field, undaunted. Afterwards, Mr Hoon received a slew of e-mails, some from Labour members, protesting about his treatment of the referendum supporters.

These Labour rebels resent being accused of disloyalty. All they are doing, they protest, is standing by a manifesto commitment. It is the others who are rebelling. And this argument will have traction in the House of Lords too. Tonight's vote is unlikely to lead to a defeat for the Government, but it is perfectly possible that the Lords will vote for a referendum in the early summer. True, there are a lot of former ambassadors and Europhile Tories there; but equally, the remainder aren't likely to fall for the line that the treaty is substantially different from the constitution. If enough crossbenchers support a referendum, it will pass.

Then the Government will be in trouble. The usual ping-pong will take place, but the unelected Lords cannot be expected to give in on the ground that the legislation was in the manifesto - quite the reverse. Could Mr Brown then resort to the Parliament Act in order to prevent the public having a say on an unpopular measure? He could, but it would be disastrous for his reputation.

Last time this matter arose, after the constitution was agreed, Jack Straw, who was then the Foreign Secretary, tried to persuade Tony Blair to hold a referendum. Mr Blair initially resiled, but after a few months, Mr Straw tried again, this time arguing not on the principle but on the arithmetic. He explained that Labour was going to lose seats in the 2005 election anyway because of Iraq. It would lose more if the Government refused to hold a European referendum. Then, after the election, there would be enough rebels to force a referendum anyway. So why not give in now?

We are not that close to an election yet. But if the Lords were to support a referendum and the issue were eventually pushed through under the Parliament Act, with the requisite one-year delay, that would take us neatly to the next European elections and, possibly, a general election.

Already voters are telling pollsters that they are less inclined to vote for MPs who fail to support a referendum. This story won't end tomorrow night. It could run and run.

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