After a fortnight of trailing heavily against George Bush, the Democratic challenger John Kerry appeared to have recovered much of the lost ground in two polls.

Another, however, showed Mr Bush pulling away from Mr Kerry with a 13 percentage point lead.

Mr Bush got another boost last night when Florida's top court ruled that the independent candidate Ralph Nader should be allowed on to the ballot, following a dispute. Mr Nader is considered likely to lure support from Mr Kerry.

Voter volatility was best summed up by two polls for the Pew Research Centre. The first, between September 8 and 10, gave Mr Bush a 16-point lead. The second, conducted between September 11 and 14, had Mr Bush with a statistically insignificant one-point lead among probable voters and the two candidates were deadlocked among registered voters.

A Harris Interactive poll, conducted from September 9 to 13, gave Mr Kerry a one-point lead - the same organisation gave Mr Bush a 10-point lead in June. A Gallup poll published yesterday and conducted from September 13 to 15 showed Mr Bush widening his lead with a 13-point advantage over Mr Kerry.

Given the low turnout, the difference between likely voters and registered voters could be a significant factor.