By Jon Swaine, Telegraph.co.uk
Instant polls taken at the end of the event by television channels suggested that despite Mrs Palin managing a more measured performance than many had anticipated, voters were more impressed with Mr Biden.
A poll by CNN and Opinion Research found 51 per cent thought Mr Biden had performed best, with Mrs Palin taking 36 per cent of respondents.
A survey of uncommitted voters for CBS found the split even more pronounced, with Mr Biden on 46, Mrs Palin on 21 and a relatively high proportion stating that neither had impressed or that they could not decide.
When the uncommitted voters were pressed, 18 per cent said they would vote for Barack Obama and Mr Biden following the debate, with only 10 per cent ready to line up for John McCain and Sarah Palin.
Another instant poll done by Fox News, which did not allow for such fence-sitting, had the divide between the two candidates at 61 per cent for Mr Biden and 39 per cent for Mrs Palin.
However a focus group of independent voters gathered by the pollster Frank Luntz, whose analyses on BBC Newsnight have proved influential in British politics in recent years, called the debate in favour of Mrs Palin.
Among the group, half of whom voted for the Democrat John Kerry in 2004, an overwhelmingly majority raised their hands to signal Mrs Palin had won the debate when asked.
While about a dozen said that Mrs Palin had changed their minds and that they would now support Mr McCain, only one person in the group said he had changed his mind in support of Mr Obama.
Other websites ran less scientific measures of voter reaction. A poll by 3,887 readers of Forbes.com found that 73 per cent believed the debate had been won by Mr Obama, with just 23 per cent plumping for Mrs Palin.
However, a similarly rough survey of almost 300,000 readers of the Drudge Report website made the split 69 per cent to 29 per cent in favour of the Republican candidate.