The political impact of TV debates in other Parliamentary democracies (and yes, yawn yawn, obligatory American reference, in the US too) has often been more about expectations than about absolute performance. Beat expectations and you benefit from the debate, even if that means people viewed you as the narrow loser. But if you were expected to be a big loser and then beat expectations and only just lose, you benefit.

Also the impact of debates has often been to reinforce people’s existing predilections rather than switch people between different parties or candidates. That has, for example, been a common feature in Canada, where TV debates have been held off and on since 1968. (Yawn yawn, US example, 1988 second Dukakis-Bush debate and others.)

In other words, you’re best placed to come out well from a debate if your party is the one most in need of motivating its supporters and if the expectations about your performance are low. Step forward then, Gordon Brown.

As for Nick Clegg?

Both of Brown’s advantages are advantages over David Cameron – and only over David Cameron. Liberal Democrat share of the vote is fairly static overall as turnout changes: from purely partisan motives, the level of turnout does not really matter, though of course from the perspective of health democracy higher turnout is much to be preferred. The expectations one is trickier, but the expectations amongst many in the media that Nick Clegg will benefit hugely from being in the debates is based on simply him being there, so he won’t go in to them with the pressure of extremely high personal performance being expected by the media.

Moreover, for Nick Clegg there is that third factor: TV debates can raise the profile of leaders beyond the main two parties.

For Nick and the Liberal Democrats this is likely to be a major boost, because consistently the party does best when it is in the news (even if, during the post-Kennedy leadership contest several MPs did their level best to disprove that). As simple a move as asking people about their views of party leaders before asking them which party they’ll vote for raises the third party’s vote in opinion polls. That’s why for many years Gallup gave the party higher ratings that other pollsters.

Compared to that, appearing in a trio of TV debates alongside Brown and Cameron will be a massive boost for Nick Clegg and the party.

Whilst we wait to find out what the televised general election debates will bring, enjoy this moment from the 1988 Canadian debates. The 1988 election was a re-run contest between Brian Mulroney’s Conservatives, who had won a landslide in 1984, and the Liberals under John Turner, still leader despite leading his party crashing out of power in 1984. John Turner is the silver haired one: