This is a good moment to be a Liberal Democrat. Constitutional reform grips Westminster. The bigger parties are being discredited by expenses - the double departure yesterday of Labour's Margaret Moran and the Tories' Julie Kirkbride symbolising shared disgrace. Lib Dems have been tainted, too, but not on the same scale, and the party can claim to have championed reform when it was unfashionable. Nick Clegg's proposal yesterday for a 100-day revolution was eye-catching, but most of the ideas in it have been backed by his party for years. The difference is that other parties are now competing to match them.

All this ought to put a spring in Lib Dem steps. The party has had an unhappy last few years, dumping two leaders while searching for a cause to equal opposition to the Iraq war. And Mr Clegg's first year in charge did not go down well with voters. David Cameron took the market for fresh-faced public schoolboy leaders; Gordon Brown looked like the heavyweight progressive. No one seemed quite sure what Mr Clegg wanted to be. Vince Cable cut through the economic crisis with an intelligent mix of diagnosis and cure. But that did not help Mr Clegg build a public personality of his own. He seemed nice, but a bit anonymous.

This is changing. The Lib Dem leader pushed for Gurkha rights, and was rewarded when MPs passed a Lib Dem motion in the Commons for the first time in decades. He has run a bold pro-European campaign in the current elections (though no one has noticed). Labour's woes allow him to pitch for progressive votes. He is putting his energies into building a base in urban Labour heartlands in northern cities, in the hope of ending his party's dependence on winning Tory votes in remote rural seats. The Lib Dems might almost claim to be Britain's only national party: able to compete in Sheffield or Liverpool as easily as they do in Truro.

So there is plenty of scope for self-congratulation. The Lib Dems have been ahead of the curve on the three great contemporary crises: climate change, the constitution and the economy. That is quite an achievement for a small party which struggles most of the time to get reported. But it brings two linked problems. The first is that voters may not reward the party just for being sensible. Next week's elections could go badly. The Lib Dems, who face losing their two county councils - and some MEPs, thanks to a cut in British representation. But more than that the party needs to define its strategy. Mr Clegg does not want to lead an insurgent thinktank. He talks of replacing Labour as a progressive force. It could happen. But the search for the elusive breakthrough is the ancient curse of third-party politics.