There is little space for third parties in a two-party system. The Liberal Democrat party, the UK’s third political bloc, is usually confined to brief cameo roles. But now, as the ruling Labour party crumbles, they believe they have a chance to seize centre-stage.

Nick Clegg, the leader of the Liberal Democrats, has said that it is “not beyond the realms of possibility” that his party could move into second place at the next election, ahead of Labour. His party might well receive more votes than Labour; they nearly managed that feat in 1983.

But the Lib Dems are hamstrung by the distribution of their base. Even large increases in their support will mean they come second in more places, rather than winning many more MPs. It is quite possible that the Lib Dems could amass more votes than Labour nationally and yet return 100 fewer MPs to the Commons.

In truth, a much more likely way for the Lib Dems to gain power is a hung parliament; despite the current large Tory opinion poll lead, investors have estimated the probability of neither main party winning a straight majority in parliament at around 25 per cent.

This leaves the Lib Dems in an awkward position. Their election campaigning will be dominated by questions of whether they would support Labour or Tory minority governments. But they must not be distracted by these matters.

First, it would be political suicide for them to be seen as proxies for either of the two other big parties. Second, by offering the electorate a distinctive platform, they play an important role in the UK’s policy debate.

Of the big national parties, only they opposed the war in Iraq and only they have consistently opposed Labour’s illiberalism. Thanks to Vince Cable, their economic spokesman, the Lib Dems have also been prominent in the debate on the financial crisis.

At the moment, they are also leading the debate on the country’s fiscal dilemma. Whereas the main parties continue to exchange bromides, the Lib Dems have made the bold, if contentious, decision not to support renewal of the UK’s Trident nuclear deterrent.

Offering clarity when others prefer obfuscation is a valuable public service. Even if the Lib Dems do not win, they flush out the inconsistencies of their opponents. In their eagerness to become one of Britain’s governing parties, the Liberal Democrats must avoid becoming risk-averse. Their ultimate selling point is they are not like the other parties.