Americans are split on whether they think the invasion of Iraq was justified. When the bombing began, approval ratings for the invasion were as high as 80 per cent.
'We've been polling since October,' said Adam Clymer, political director of the Annenberg Election Survey.
'I think [the invasion] is likely to be very important in the election. Whether it is more important than the economy - those will be the two dominant subjects.'
Support for the invasion 'perked up' in January after the capture of Saddam Hussein, Mr Clymer said, adding the lower percentage of support has been a steady trend, because those who initially supported the invasion did so on the grounds 'Iraq posed a threat to the United States'.
Frustration has also surfaced because no weapons have been found in Iraq and a clear link to al-Qaeda has not materialised.
However, Mr Clymer said the recent violence in Iraq and Spain did not seem to have an effect on support for the invasion.