The final Reuters/C-SPAN/Zogby poll of the election published on Tuesday found likely voters favoured Mr Obama by 11 points over Mr McCain, 54-43%.
Other national polls indicate Mr Obama increasing his lead over his rival to as much as 13 points.
But the BBC's James Coomarasamy in Washington says that while Mr Obama has held a consistent lead for several weeks, a number of factors could undermine the pollsters' predictions.
Among them, he says, are the role the Illinois senator's skin colour may play in voters' intentions; whether newly registered voters will actually vote; and the Palin effect - whether Mr McCain's running mate has energised or alienated Republicans.
Under the US Electoral College system, states are allocated votes based on their representation in Congress.
In almost every state, the winner gets all these college votes.
To become president, a candidate needs to win a majority across the country - 270 college votes out of a possible 538.
BBC North America editor Justin Webb says there is much interest in three well-populated swing states - Florida and Ohio, both won narrowly by George Bush in 2004, and Pennsylvania, which went to the Democratic candidate John Kerry.
If Mr Obama can take Florida or Ohio, he is sure to become president, our correspondent says.
If John McCain holds them and takes Pennsylvania, he could just win, our correspondent adds.
The presidential election has been the most expensive in US history - costing $2.4bn, according to the non-partisan Center for Responsive Politics.
On the eve of the poll, Mr Obama said his grandmother, Madelyn Dunham - who largely raised him as a child - had died aged 86 in Hawaii after losing her battle with cancer.
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