Henry Farrell makes a prediction:
My best guess - as discussed before - is that the initial boost for the Liberal Democrats from the first debate was less a simple result of Clegg having done well, than of Clegg suddenly appearing to be a viable candidate, voters (who might have preferred to have voted Lib Dem if they didn’t think their vote was going to be wasted) seeing that others perceived Clegg as a possible winner and revising their own voting tactics etc. In other words - the debate didn’t pick winners or losers based on the strength or weakness of their performance so much as it revealed possibilities that voters had hitherto discounted.
If this is right, we will not see a major boost for Cameron in the final vote. Instead, we’ll see a continuation of the trendline - Labor doing badly, the Liberal Democrats doing well, but bleeding some support, and the Conservatives doing fine, but not fine enough to win a majority in Parliament.
Dan Berman and Renard Sexton have a slightly different read:
It seems that Clegg’s vacillation over the weekend as to who he would back in a hung parliament was at least part of his undoing. In the second debate he had been able to pretend that he was a serious contender for the big chair, confidently stating that the Liberal Democrats in power would do this or change that. The coalition discussions ended that. By the time of the third debate, it was clear that though one man on the stage would end up Prime Minister, it would not be Clegg.