And in the latest twist to the campaign saga, the Associated Press news agency reported on Saturday that Obama's aunt has been living in the US illegally for the past four years after her request for asylum from Kenya was rejected.
The latest opinion poll by Reuters/C-SPAN/Zogby indicated Obama's lead nationally had narrowed slightly to 49.1 per cent, compared to McCain's 44.1 per cent.
Previously Obama's lead had been at 50.1 per cent to McCain's 43.1 per cent, with both sets of results having a three percentage point margin of error.
The Obama campaign on Saturday meanwhile was swift to seize on the news that Dick Cheney, the current Republican US vice-president, had endorsed McCain for president.
"Do you think Dick Cheney is delighted to support John McCain because he thinks John McCain is going to bring change to Washington? Do you think John McCain and Dick Cheney have been talking about how to really shake things up?" Obama told a rally while campaigning in Colorado.
'Politics of fear'
McCain launched another attack on Obama's national security credentials during a speech in Virginia on Saturday.
"The question is whether this is a man who has what it takes to protect America from Osama bin Laden, al-Qaeda, the prospect of a nuclear-armed Iran, and other grave threats in the world," McCain said.
"And he has given you no reason to answer in the affirmative."
Obama, meanwhile, who has been campaigning in Missouri, Nevada and Colorado, warned that despite his lead in the polls, there was still work to be done to win the race for the White House.
"Don't believe for a second that this election is over," Obama said in the western battleground state of Nevada.
Rob Reynolds, Al Jazeera's senior Washington correspondent, in Colorado, said Obama was targeting traditionally Republican states in the hope of taking some key electoral college votes in areas that are not traditional swing states such as Ohio and Florida.
|US voters have already taken part in
early polling across the country [AFP]
Al Jazeera's Rosiland Jordan, reporting from Springfield, Virginia, said the state is a must win for McCain and one he should not even be fighting in, as it has not voted Democratic since the 1960s.
However, the latest polls put him at 6.5 per cent behind Obama there.
"If you read the polls closely, you do see a point movement into positive territory for McCain," Jordan said.
But the crowds are not what they are for Obama and if you take a look at early voting, it seems more Democrats are going to vote [nationally]."
Under the US political system, the president is elected not by direct popular vote but
by capturing 270 out of 538 electoral votes distributed throughout the country in a state-by-state contest.
On Saturday the AP quoted a statement from Obama's campaign as saying that Obama had "no knowledge" of his aunt Zeituni Onyango's status but that he "obviously believes that any and all appropriate laws should be followed".
Obama's campaign said it had also returned $265 in donations from Onyango.
Reynolds said the revelation "could become something of an embarassment to the Democratic candidate because it could amplify or accentuate in the minds of some people this notion that ... Obama is not truly American".
However he added that there were concerns in the Democratic camp that the leak had been engineered for political purposes.