There have been two very different audiences for the UK leaders' debates. First there are the already committed, eager for their man to win and inclined to believe he has done so. If they come to the conclusion he has not, perhaps they might waver, but otherwise their loyalty is simply reinforced.
Then there are the uncommitted, still not quite sure which party they are going to support, if any. Some of them might only be watching by chance. Yet it is their votes that are most likely to be influenced by what they see and hear.
The former is by far the larger of the two groups. So unless one of the contenders clearly outperforms his rivals – as happened in the first debate – it's only to be expected that polls asking people who they think won the debate largely reflect the current popularity of the parties in the polls.
On average those polls conducted wholly since Monday have put the Conservatives ahead on 35 per cent, while the gap between Labour and the Liberal Democrats has apparently closed with both running at 28 per cent.
So it should not have come as much surprise that Mr Cameron emerged as the winner in the instant post-debate polls. On average the five such exercises found that 37 per cent thought Mr Cameron had come off best.
Somewhat more surprising, therefore, is the fact that Mr Clegg came second, well ahead of Mr Brown. As many as 32 per cent thought he was best – little different from the 33 per cent who did so last week – while only 26 per cent thought Mr Brown came first.
Although the debate focused on what was meant to be his specialist subject, no less than 50 per cent told Populus they thought Mr Brown had come off worst, well above the 42 per cent who held that view last week.
But in each case, as expected, those who were already inclined to support a particular party typically reckoned their man had won. Thus ComRes found that 73 per cent of Conservative supporters reckoned Mr Cameron had won, 64 per cent of Liberal Democrats thought Mr Clegg had won, and 68 per cent of Labour supporters thought Mr Brown had won.
What, though, of the crucial group of uncommitted voters? Here, a rather different picture emerges. Among this group it appears it was Mr Clegg who scored a narrow victory. On average, across the four polls that have published the necessary details, 34 per cent thought Mr Clegg did best, while Mr Cameron's score of 27 per cent was even lower than Mr Brown's 28 per cent.
Moreover, there are some signs that Mr Clegg's performance may have had most success in persuading the uncommitted to vote for him. According to Populus, a far higher proportion of the uncommitted say they are now more likely to vote for the Liberal Democrats than say they are more likely to back the Tories or Labour.
Not that the debate will necessarily produce much of a boost for the Liberal Democrats in overall voting intentions. According to ComRes, overall voting intentions among those who watched the programme were virtually unchanged from last week.
But if Mr Clegg's performance has helped stabilise a Liberal Democrat vote that appeared to be slowly sagging, then he will have helped ensure his party remains in the race for second place in votes in the final week of the campaign. If so, then the mould of British politics is still looking rather fragile.
John Curtice is professor of politics at Strathclyde University