Ahead of the most intense two-and-a-half month drive to election day in recent memory, Democratic hopeful Mr Obama holds only slim leads in national polls and in several important swing states.
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A Los Angeles Times/Bloomberg poll released on Tuesday gave him a slim lead of 45 to 43 per cent, within the survey's three point margin of error. In June, he had led by 12 points, though other polls at the time had the race slightly closer.
A new poll by the Quinnipiac University, south of Boston, also showed Mr Obama's national lead over Mr McCain slipping – he led 47 to 42 per cent, down from a 50 to 41 per cent lead nationally in the same poll a month ago.
The new surveys reveal doubts about whether Mr Obama is ready to be president. His pick of a running mate is imminent and it's a week before he is due to be formally crowned the Democratic nominee.
Mr Obama will hope that television coverage of his vice-presidential nominee selection within the next few days, and the Democratic National Convention in Denver will give him a polling “bounce”.
'Lack of experience'
Mr McCain will seek a similar boost from his vice-presidential pick and convention the following week.
A RealClearPolitics average of national polls gave Mr Obama a three point lead, down two per cent from a month ago, while a CNN poll of polls average had Mr Obama up by four points on Tuesday.
The LA Times/Bloomberg data suggested some voters were still troubled by first term senator Mr Obama's lack of experience, and revealed a slump in his favourable ratings to 48 per cent from 59 per cent in the last such poll in June.
When voters were asked if Mr McCain had the right experience to be president, 80 per cent said yes, while 48 per cent said Mr Obama lacks the necessary experience for the job.
There were also lingering questions about Mr Obama's love of country. Eighty-four per cent of those polled found Mr McCain strongly patriotic, while only 55 per cent said the same of Mr Obama.
Polling in several battleground states also appears to be showing an erosion in Mr Obama's support over the last month.
On Wednesday next week he will be crowned the party's standard-bearer to take on Mr McCain in November's election, and will give his acceptance speech the next day before up to 80,000 supporters in a Denver sports stadium.
While the convention will proclaim party unity, disaffected supporters of Mr Obama's defeated rival Hillary Clinton plan to put on a show of protest. But the new champion said talk of Democratic division was “hyped by the media”.
“We had a vigorous campaign. Senator Clinton ran a historic campaign,” he said, adamant the former first lady was now fully on board his shot at becoming America's first black president.
Meanwhile, Mr McCain, battling to carve an opening on the economy, flew to a Gulf of Mexico oil rig 220 kilometres off New Orleans to demand expanded offshore drilling.
“Senator Obama opposes new drilling, he said it won't solve our problem and that it's 'not real' – he is wrong and the American people know it,” the Arizona senator said.
“The nation is sending 700 billion dollars every year overseas to (oil-exporting) countries that do not like us very much. When I am president that is going to stop.”