By David Cowling
Editor, BBC Political Research Unit
Expectations differed for each candidate ahead of the first debate
A number of opinion polls are being released in the lead up to the election. This week, the impact of the first televised prime ministerial debate on the electorate is assessed.
The latest, a YouGov poll for Monday's Sun newspaper puts the Lib Dems ahead on 33%, Conservatives on 32%, Labour on 26%.
The normal margins of error make it unclear whether the Lib Dems or the Conservatives are really in first place but there's no dispute that Labour is third.
This poll does not move us on much from Sunday's polls as regards voting intention, but the additional questions set out the battleground between now and polling day.
These questions show significant levels of public opposition to a whole raft of specific Lib Dem policies and many voters not knowing what Nick Clegg stands for.
We can be sure that both Labour and the Conservatives will do their best to educate them in the weeks ahead.
If the voting intention figures for this poll are put through the BBC Online's election calculator then, on a uniform swing, the Conservatives take 246 seats, Labour take 241, the Lib Dems 134 and other parties 29.
A ComRes poll for the Sunday Mirror and Independent on Sunday puts the Conservatives on 31, the Lib Dems on 29 and Labour on 27. An ICM poll for the Sunday Telegraph puts the Conservatives on 34, Labour on 29 and the Lib Dems on 27.
The ComRes telephone poll and YouGov internet poll were sampled after the first prime ministerial debate on Thursday.
The Conservatives have taken a bigger hit than Labour but the three parties are so close that margins of error should caution us about bold declarations of who is clearly in the lead, or clearly in third place.
What both polls appear to confirm is the extraordinary shift in Lib Dem support as a result of the first prime ministerial debate.
Doubtless the reasons for this will be discussed endlessly.
However, as far as the polls so far published are concerned, we have no historical comparisons to help us judge whether this turbulence will now dominate the campaign, or whether on polling day far fewer people change their votes because of the debates than say they might now.
Clearly we should reflect on the story the polls are telling us but, before we are tempted down the path of declaring a new dawn in British politics, remember that the polls might just be reflecting instant reaction to the new phenomenon of the debates.
There are two more debates to go where today's pattern could change. Nor should we forget that both the earlier YouGov and today's ComRes polls, on a uniform swing, would leave Labour as the largest party in a hung parliament.
ICM/Sunday Telegraph was sampled largely before the first debate. To that extent it does not offer us a clear picture of the full extent of public reaction but even so it still registered a six point increase in support for the Lib Dems.
A Sun newspaper poll, carried out after the TV debate, suggests Labour are in third place on 28% (down 3%), with the Lib Dems on 30% (up 8%) and the Conservatives 33% (down 4%). Applying the figures from The Sun poll, which came from a YouGov survey of 1,290 people, to the BBC News website's election seat calculator, results in the following: Labour 276 seats; Conservatives 245 seats; Lib Dems 100 seats; Others 29 seats.
This is the first national poll sampled after Thursday's debate. Clearly the findings, if confirmed by the polls expected on Sunday, are important. It is worth pointing out that the two point difference between the Liberal Democrats and Labour is within normal sampling error, so it does not mean Labour is definitively in third place.
Although the headline focus may be on the Liberal Democrats appearing ahead of Labour we should not ignore the poll's suggestion that the Conservative fall in support is bigger than Labour's.
Perhaps it is best to consider this single poll as an immediate referendum on Thursday night's debate, until we have evidence, if any, that it represents the settled will of British voters.
THE FIRST WEEK'S POLLS
The first week has shown the telephone random sample polls with the following ranges between the main parties: Labour 29-33%, the Conservatives 35-39% and the Lib Dems 16-21%.
Among the internet pollsters who draw their samples from panels of respondents, the ranges have been: Labour 26-33%, the Conservatives 37-41% and the Lib Dems 17-22%.
The difference between them therefore is a slightly stronger performance by the Conservatives in the internet polls and a wider range for Labour support in the same polls.
The distinctive feature of most of them is that they suggest, on a uniform swing, that we are in hung Parliament territory.
But the 12 April Populus/Times poll found 50% of respondents hoping for a clear majority government to emerge after 6 May.
I find this one of the most difficult elections to call in decades. Labour have been on the ropes for quite some time but the Conservatives in recent months have found it very hard to translate Labour unpopularity into strong positive support for themselves.
The same Times/Populus poll illustrated this uncertainty for me.
Some 42% of respondents said it seemed time to change from Labour but they were not sure it was time to change to the Conservatives. Only one in three agreed without qualification that it was time to change from Labour to the Conservatives.
Perhaps one factor contributing to this uncertainty was revealed in the ComRes/Sunday Mirror/Independent on Sunday poll, conducted on 9 and 10 April.
This found 63% agreed with the statement that "neither Labour nor the Conservatives are being honest about how they would reduce public spending".
However, most recent campaign polls have been viewed as simply marking time until the first of the prime ministerial debates. In the game of expectations, Mr Cameron had won hands down days before Thursday's event.
When ComRes/ITV News/Independent asked people on 11 and 12 April which of the three leaders was expected to win the debate: 42% said David Cameron, 22% Gordon Brown and 10% Nick Clegg.
So, ironically, the pressure for this event was not on the prime minister but on the leader of the opposition.
In the frenzy of instant post-debate polling, the one unambiguous verdict was that Mr Clegg had won by a country mile.
These figures are based on samples of people who watched the debates. However, about 36 million people on the electoral register did not.
There will be a clutch of polls on Sunday that should give us the first reliable indications of how far, if at all, the debate has influenced voting intention across the entire electorate.
But we also need to be careful about all the post-debate hype.
On 6 May, there will be only one constituency in Britain where Nick Clegg's name will appear on the ballot paper and about 60,000 out of 46 million electors will have the opportunity to vote for him.
In the other 649 constituencies, decisions about who to vote for will be made, as usual, for a variety of reasons, one of which may be the standing of the respective party leaders.
However, without doubt, the pressure is now on Mr Brown and Mr Cameron as they contemplate the two debates to come. Mr Cameron has lost most simply because so much was expected of him.
If Mr Clegg's success in the debate feeds through into significant changes in the opinion polls, the outcome on 6 May will become even more difficult to predict.