The two leading candidates have intensified their campaigns with just two weeks to go before the crucial vote on November 4.
On Tuesday, McCain spoke in Pennsylvania, one of the only Democratic-leaning states in which the Republican candidate is still aggressively campaigning, while Obama continued campaigning in Florida for a second day.
Obama held a 'Jobs Summit' in Fort Worth Florida alongside two of his highest profile supporters in the business world - Eric Schmidt, the head of Google, and Paul Volcker, the former head of the Federal Reserve.
Al Jazeera's James Bays in Florida said the meeting was an attempt by Obama to show that he could offer solutions to US economic woes alongside his criticism of Republican policies.
The Illinois senator's healthy lead in polls in Iowa and Wisconsin could mean that cancelling his appearances there would not seem to pose much of a political risk.
Obama lauded Madelyn Dunham, his grandmother, as playing a key role in his life in his convention speech in August.
"In the last few weeks her health has deteriorated to the point where her situation is very serious," Gibbs told reporters on Obama's plane in Florida.
For the second consecutive day, McCain noted the weekend prediction of Obama's running mate, Joe Biden, that an international crisis would test Obama during his first six months in office.
"We know senator Obama won't have the right response," the Arizona senator said in Bensalem, Pennsylvania.
On Monday, early voting began across the US, with millions casting their ballots by post and at special early voting polling stations, although the results will not be published until the full poll takes place.
The early polls saw record turnouts, according to election officials.
In areas known to be heavily Democratic in the states of Georgia, Iowa, North Carolina and Ohio, people have been requesting and submitting ballots in large numbers, the Associated Press news agency reported.
Ebonee Lusk, who voted early in Fort Wayne, Indiana, said she could not wait until November 4 to cast her ballot.
"I wanted to get in, cast my vote for Barack Obama and make sure my vote counts," she said.
In Florida, Republicans appeared to hold the edge in early voting, and McCain's campaign has said it is hoping to catch up with Obama by election day.