Meanwhile, McCain held a rally in Miami, Florida, on Wednesday before meeting US military leaders for discussions on national security issues.
Florida, which has 27 electoral college votes, voted Republican for the past two elections but recent polls indicate that Obama is tied with McCain.
The candidates need to capture 270 college votes out of 538 in order to win the US election, as the national popular vote does not determine the winner.
Al Jazeera's Rosiland Jordan in Miami says Florida is a "must-win" state for McCain, with 10 per cent of the votes needed to win the election, and his camp has been telling supporters not to trust the polls as they claim they can pull off a surprise win in the state on November 4.
The AP-GFK poll, released on Wednesday, has Obama leading in the state of Ohio by seven percentage points, in Nevada by 12 points, Colorado by nine points and Virginia by seven points.
All were previously Republican states won by George Bush, the current US president, in 2004's election.
In addition, the poll shows Obama is tied with McCain in North Carolina and in Florida, two states also carried by Bush in 2004.
Nationally, a poll by the Pew Research Centre on Tuesday gave Obama a 16-point
lead among registered voters, with 52 per cent compared to McCain's 36 per cent.
Other nationwide polls have shown Obama holding a lead in single digits.
Obama has saturated US media outlets with advertisements in recent days and has bought 30 minutes of advertising space to run on three major US television channels on Wednesday.
Costing millions of dollars, the ads are expected to focus on the economy.