Blog: At this stage of the campaign last week, I looked forward to the first of the leaders’ television debates – and miserably failed to forecast the impact of Nick Clegg’s performance, writes John Hipwood.

I suggested that it was hard to consider the Lib Dem leader’s policies in “the real world” because, like most other people, I didn’t expected him to be in any position to implement them.

All that’s changed now following the extraordinary public reaction to the first debate. Mr Clegg is now a real player in the pre and possibly post-election world of politics in this country.

Whether he deserves to be after one dynamic performance on TV is not the point. He’s there, and both David Cameron and Gordon Brown should fear his continuing impact if he maintains his form in tonight’s debate.

Labour and the Tories are spinning vigorously that Mr Clegg is vulnerable on foreign policy, the subject of this week’s debate on Sky, especially over his lack of support for replacing Trident and his fondness for the European Union.

But his argument might have a resonance that the like-for-like replacement of Trident, built to flatten Moscow or St Petersburg in the event of the Cold War turning into a nuclear war, should at the very least be thrown into the defence spending melting pot at a time when Britain needs to find savings on an enormous scale.

Europe is tricky for the Lib Dem leader, but it is too for Messrs Brown and Cameron. The public feel they were let down by both men in relation to a referendum on the Lisbon Treaty. And then there are those strange bedfellows Mr Cameron has joined up with in the European Parliament.

Mr Clegg has one significant ace up his sleeve – his opposition to the war in Iraq. A majority of the public are definitely on his side on this issue. Both Mr Brown and Mr Cameron supported the war.

The Lib Dem leader will find it very difficult to live up to the billing created for him by the pollsters and the media over the last seven days, but it’s getting to the stage when Mr Cameron must break through and start convincing the electorate that he’s a worthy challenger.

Mr Brown should be more comfortable talking about foreign affairs – he’s been playing on the international stage for years, and would have us believe that he saved the world with his response to the banking crisis. But this doesn’t mean he will be. Comfortable isn’t a word that sits easily with the Prime Minister. Let’s hope tonight’s debate proves to be as lively as the one yesterday when charges and insults were zapping around like unearthed electricity in exchanges between Chancellor Alistair Darling, shadow chancellor George Osborne and Lib Dem spokesman Vince Cable.

Stirring things up was Andrew Neil, but tonight Adam Boulton will be constrained by the rules designed to ensure that no leader falls foul of persistent tough questioning from the “moderator”.

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Illuminating to see Mr Cameron’s reaction to being hit by a protester’s egg yesterday. After being pursued by a giant chicken for most of the campaign, he joked:

“Now I know which came first – the chicken not the egg.” John Prescott reacted rather differently with a right hook when he was hit by an egg during the 2001 campaign.