A sound bite, a photo opportunity and a Government defeat in the Commons: Nick Clegg is suddenly a player. He took a high moral tone against the Prime Minister: "If someone is prepared to die for this country, surely they deserve to live in this country." He posed with Joanna Lumley and David Cameron outside afterwards. Then, for the first time since the days of James Callaghan, the Government lost a vote on an Opposition motion – this one tabled by the Liberal Democrats.

Suddenly, I was taken back to Saddleworth moor, to memories of hanging around the market in Littleborough, interviewing potential voters, for The Independent. The question that I was trying to answer in that by-election campaign was not whether John Major's Government would lose the seat, but whether Chris Davies would win it for the Liberal Democrats, or Phil Woolas for Labour. My straw poll in the market was inconclusive, but the Lib Dems won a famous victory and sang "Walking in a Liberal Wonderland" into the early hours of the morning.

Well, last week was Nick Clegg's Littleborough and Saddleworth moment. That by-election in 1995 was a speed bump on the road to Tony Blair's landslide. It was the moment that it became clear that Paddy Ashdown's party would resist the New Labour juggernaut. Two years later, Blair won his landslide, but Ashdown more than doubled the Lib Dem representation in the House of Commons at the same time. Many years later, it was never likely that the Lib Dems would simply fold their tents in the face of Cameron's advance. For one thing, Cameron's liberal Conservatism is a feebler thing than New Labour in its first flush. But some did wonder, with the subsiding of Iraq as an issue and Cameron's outflanking manoeuvres on the environment and civil liberties, whether we had seen the high-water mark of the third party.

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I never thought that was likely. The Lib Dems do not seem to need policies, or even a leader. Theirs is such a well-established brand that they provide the default option for any voters repelled by either of the two larger parties. And at the moment the Government is disliked while the official Opposition is still flawed.

The victory on the rights of Gurkhas showed that Clegg – as Ashdown did in 1997 – can maximise that opportunity. Although the larger question was perhaps how on earth Gordon Brown managed to misjudge the issue so badly. One Labour MP said to me: "I can't believe we did that, saying money was more important than these people prepared to die for us." Some Blairites were calling semi-sarcastically for the return of Damian McBride – "He would have told Gordon on Monday that he was on the wrong side of the issue" – as if Brown Central never made mistakes when McBride was at Brown's right hand.

But half the skill of politics is making the most of your opponents' mistakes. And Clegg showed himself to be both aggressive and articulate.

What is interesting, though, is not that Clegg is able to take advantage of Brown's vulnerability, but that the Tories also remain vulnerable. Clegg does not want to repeat Ashdown's "success" in gaining seats as part of an anti-government landslide. He needs Labour to recover so that the election is a close-run thing. Only that way can he be sure of a share in power.

Fortunately for him, Ashdown was talking his usual punk politics in his interview with The Daily Telegraph yesterday. "The situation is incredibly fluid," he said, as he always has. He spoke of the possibility of another "flip", as when "Labour usurped the Liberal Party in the early part of the last century", as he always has. I don't think so. For the same reasons that I think that Labour should get Alan Johnson in as prime minister, I think Clegg's best hope of influence is that Labour is not yet dead while the Tories are still weak.

Take just three examples from last week. The Tories launched their campaign for next month's European elections by calling for a referendum on a Treaty about which most people have forgotten and that is in limbo until the Irish and Czechs say otherwise. A funny way to "love-bomb" the Lib Dems. Clegg is privately scathing about William Hague, shadow Foreign Secretary and arch Eurosceptic.

Cameron's rather good speech about the new "age of austerity", is contradicted by his policies – notably the inheritance tax cut for millionaires. Austerity for whom? To which Cameron's answer seems to be that it is all right to cut taxes for one group of rich people as it is funded by tax rises on another group of rich people – non-doms. He needs to do better than that.

Finally, there is MPs' pay and expenses. Yes, Brown managed to make a mess of it, but the Tories remain vulnerable. Cameron's failure to make his shadow cabinet give up second jobs came back to bite him on BBC1's Question Time on Thursday. Andrew Lansley, the Tory health spokesman, admitted that he got £24,000 – the national average annual salary – for "10 or 12 days' work" as director of a marketing company. As Saint Hilary Benn said that being an MP was a full-time job, the audience hooted at Lansley. "Come on, it's 12 days, be fair," said Frank Skinner.

If Labour recovers, by which I mean changes its leader, then the fight could be taken to the Conservatives again. Meanwhile, the appearance of Clegg and Cameron on either side of a Lumley-led coalition was deceptive. It is not just on Europe that the Lib Dems find themselves, still, closer to Labour. On the economy, the Lib Dems broadly support the Government in maintaining public spending and taking a relatively relaxed attitude to borrowing. I am told that Clegg's view is that vast public debt is not a problem as long as Britain is "not considerably out of step" with other countries.

Not only does Clegg have an interest in the Government recovering from the defeat he inflicted on it last week, he would find in a hung parliament that he had more in common with Labour than the Conservatives.