Mary Ann Sieghart: This Coalition is going to last and last

The veteran publisher George Weidenfeld likes to tell the tale of his wedding to Barbara Skelton, femme fatale and serial wife and lover. Wandering around the reception after the ceremony, he kept overhearing his guests murmuring, "It won't last, of course." They were right: the marriage broke up after a few years.

At Westminster, you can hear the same gloomy prediction about the Coalition. It will break up next year when the Liberal Democrats lose the referendum on electoral reform. Or it will fall apart when the Lib Dems realise that they can't stomach the cuts. Or it will collapse when the Lib Dem poll rating falls so low that it is barely statistically significant.

This time, though, the pessimists have got it wrong. The current political pairing may look as unlikely as the Weidenfeld/Skelton marriage, but it has a greater chance of success. And, however despondent some Lib Dem activists feel this week in Liverpool, the success isn't doomed to redound to the Conservatives at the expense of the junior partner.

In fact, you would have to be a pretty Eeyore-ish Lib Dem not to be celebrating at this first conference after the election. Last September, Nick Clegg was derided for talking about what a Lib Dem Cabinet would do with power. Now who's laughing at the Deputy Prime Minister and his four Cabinet colleagues? The party has its much longed-for referendum on changing the voting system. Granted, things aren't perfect, and it has had to make common cause with the Right not the Left, but hey, that's coalitions for you. Only a majority Lib Dem government would be perfect for these activists, and you don't get that with 23 per cent of the vote.

Yes, the party's poll ratings have fallen by roughly 10 points since the election. But they always do. The Lib Dems routinely score in the mid-teens between elections and rise to the early twenties during the campaign. Yes, there are many disillusioned ex-Lib Dem voters who see coalition with the Tories as betrayal. Poor Nick Clegg has had dogshit posted through his letterbox and been spat at and shouted at in the street.

But a partnership with Labour was never possible. Any cobbled-together coalition with Labour and assorted nationalists would have involved making promises to Scottish, Welsh and Northern Irish parties to maintain public spending there. This would have infuriated the English and put intolerable pressure on the union. And the vilification that Clegg is suffering now would have been nothing compared with the accusations of political illegitimacy he would have provoked had he propped up a deeply unpopular Prime Minister whose party had lost the election.

As for allowing the Tories to form a minority government, you don't spend decades waiting for a hung parliament only to refuse to jump aboard when it finally appears. What's more, the Lib Dems couldn't have afforded – literally – to fight the second election that David Cameron would almost certainly have called this autumn. Even now, they are almost broke.

Not only was a Con-Dem coalition the best the Lib Dems could have hoped for in the circumstances; the coalition agreement was extremely favourable to them, given how few seats they won. Now they have little incentive not to see it through. For a start, coalition government is the natural outcome of proportional representation.

You can't spend several lifetimes arguing for PR only to wreck the first proper coalition that comes along for generations. The strongest argument against PR is that it leads to hung parliaments – so the Lib Dems have to prove that hung parliaments can be powerful and constructive, not doomed to weakness and paralysis. And what tribalists at Westminster – and in Liverpool – don't always realise is that voters are rather enjoying watching two parties working together in the national interest.

There is another big reason why the Lib Dems won't leave the Coalition before its time is up. If what the Government is doing is so unpopular that the smaller party can't bear it, the chances are that the Lib Dems will be doing very badly in the polls. Why precipitate another election, in that case, only to lose half your seats? Staying the course gives both parties in the Coalition the best chance to wait for the economy – and their popularity – to pick up. And for the Lib Dems, it gives them the political credibility of five years at the heart of government.

So what about the notion that if the Government does well, it will be only the Tories who take the credit? There's no reason why this should be so. As long as the Lib Dems can point to liberal policies that they have insisted upon and Conservative extremes that they have moderated, they will be able to persuade people to vote for them. These may not be exactly the same people who voted for them last time: there may be more wet Tories and fewer anti-war lefties. But that's precisely what Clegg wants. And while it's true that there is no box on the ballot paper for a hung parliament, if voters decide that they would rather like another coalition, the best way to get one is to vote Lib Dem again. The more seats the Lib Dems win, the less chance there is of one of the two main parties winning a majority.

Just because the Lib Dems have thrown in their lot with the Tories this time doesn't mean they will be gobbled up for ever. That's not how it works in the rest of Europe. Clegg has recently had a long talk with Guido Westerwelle, the leader of Germany's third party, the Free Democrats. The FDP has formed coalitions with both Christian Democrats and Social Democrats. That's what it means to hold the balance of power.

Clegg is planning to tell his activists in his leader's speech today that if they really believe in pluralism, they have to be prepared to form a coalition with either party. And if what they really believe in is realignment of the centre left, then that's not pluralism.

The Deputy Prime Minister has learned another lesson from his German counterpart. If, as leader of a minority party, you agree to run a department, you become too distracted to wield much influence over the rest of government policy. If you are Foreign Secretary, it's worse still, as you are out of the country for much of the time. Cameron kept offering Clegg jobs during the coalition negotiations, but Clegg knew that he didn't want a department of his own and refused even to discuss jobs for himself and his colleagues until the very end, after Gordon Brown resigned. He realised that it would be too easy to be bought off with Cabinet seats at the expense of Lib Dem policies.

The outcome is that the leader of a party with only 57 parliamentary seats has disproportionate influence on government. Clegg played a relatively weak hand well and – if his party is patient – has a good chance of turning this Coalition to his advantage. It's too easy to make lazy predictions like "the Coalition can't last" or "the Lib Dems will be subsumed." Politics have been transformed since May and we have to reassess our old prejudices. There is one glaring fact that the old Westminster lags have missed: the Coalition is likely to last the course because the only party that would gain from it being wrecked is Labour.

m.sieghart@independent.co.uk; www.twitter.com/MASieghart