John Rentoul: Clegg has trust issues. Sound familiar?

It would be easy to write off the Lib Dems. And well-advised. But the betrayal began long before Thursday night

Now that it is all over, Dr Hindsight has come to tell us what should have happened. It is patently obvious that Nick Clegg should not have started from where he did. We have been over this ground before, many times. Signing the pledge to vote against any increase in tuition fees was possibly the biggest error of political judgement since Nicholas Ridley told Margaret Thatcher that a charge per person for local council services was an excellent idea that would be popular.

Even if Clegg, Vince Cable and Danny Alexander had not personally signed the pledge, the manifesto on which they had been elected would still have promised to "scrap unfair university tuition fees [by] a financially responsible plan to phase fees out over six years".

But Dr Hindsight starts with the formation of the coalition government and the decision by Clegg, Cable and David Cameron that the Liberal Democrats should finesse their own promises. That was the moment of betrayal, not Thursday night in the House of Commons. It was in that first, seven-page document agreed on 11 May that the sell-out occurred. This was the sentence: "If the response of the Government to Lord Browne's report is one that Liberal Democrats cannot accept, then arrangements will be made to enable Liberal Democrat MPs to abstain in any vote." Cable was appointed Secretary of State for Business the next day. But the die was cast. That sentence meant that the Lib Dems would not block the Government's proposals, whatever they were.

So it is beside the point to complain about the 28 Liberal Democrat MPs who voted for higher fees last week. Equally, the praise for most of the 21 who voted against is undeserved. The only Lib Dem MPs who have the right to hold their heads half high are Charles Kennedy, John Leech (new MP for Manchester Withington) and the third, unknown, MP who allegedly abstained on the night of 11 May on whether to go into the coalition. Although they were hardly brave – I have a vision of Kennedy at the back of the room, pretending to scratch his head when a show of hands was sought, and then deciding in the days after to announce that he had withheld his support.

Again, at the party's special conference in Birmingham on Sunday 16 May, dissenters and abstainers were not counted or recorded. The coalition agreement was "overwhelmingly approved". So it was the overwhelming majority of MPs and party delegates who betrayed their election promises. (Strictly speaking, some Lib Dem MPs then re-ratted on Thursday, by voting against higher tuition fees when the coalition agreement provided only for abstention if they didn't like Cable's plan.)

So, Dr Hindsight, how should Vince Cable, having broken his promises, have enacted the treason? Well, that is easy, and I am glad you asked.

First, Cable should not have tried to persuade John Browne, commissioned by Labour to review student finance, to make his report more palatable to the Lib Dems. Instead, he should have conspired with Lord Browne to make the report an eye-watering argument for a higher education market. Then he could have made a great show of rejecting parts of it and replacing them with the milk of social democracy and the balm of the liberal arts. Lord Browne, I'm sure, would have understood.

In doing so, Cable should have applied the Humpty Dumpty Rule of Politics, that words mean what you want them to mean. He should have "scrapped" tuition fees by calling them a graduate account. And he should have called his scheme a graduate tax even if, technically, it isn't.

I was intrigued that Paul Waugh of Politics Home reported last week that Cable had indeed used the words "graduate tax" in a draft of his important speech in July. He was forced to take them out, either because of the Treasury's institutional aversion to new taxes or because of David Cameron's political hostility to them, or both. The small print of the legislation would have to make clear that the obligation owed by graduates was an enforceable contract repayable through the tax system, but who understands the difference between loans-for-fees and a graduate tax?

Of course, trying to disown the words "fees" and "debt" might not have worked well. The furies would still have descended on Cable and Clegg, accusing them of trying to wriggle out of their promises with semantics, but would that be worse than what they face now?

Because this looks very much like the end of the Liberal Democrats as a political force. The easiest thing to write after last week's dramas would be: don't write off Clegg. But I don't see it. All I see is a party broken by that vote. Only eight backbenchers voted with the Government. One of them was David Laws. Another was Sir Alan Beith. Proportionately, it was a far bigger rebellion than Labour's over Iraq. I see 17 ministers and three of their aides voting to keep their jobs. A leadership challenge is just gossip, but I see one minister, Chris Huhne, waiting to pounce, absent from the vote doing saintly green stuff in Cancun, the unsullied king across the Atlantic (unlike Greg Barker, his junior Tory minister, who returned to spend nine hours on British soil, a motorbike taxi waiting at the Commons while he voted).

I see a party repeating a pattern: in every peace-time coalition, the Conservatives have been strengthened, while their Liberal or Labour junior partners have been marginalised. And I see a party, after six months in government, that has "this huge stuff about trust", as Alastair Campbell wrote after six years. The only strategy that holds out any hope for Clegg at the 2015 election is for the Lib Dems to offer "radical", "progressive" or "sick" policies – whatever the buzz word is by then – that are distinctive. He can say, "Vote for us and we will influence the government if parliament stays hung." We don't need Dr Hindsight to tell us how the voters are likely to respond.;