At an 8,000-strong rally in Tampa, Florida on Monday, Obama criticised McCain, saying: "In the final days of campaigns, the say-anything, do-anything politics too often takes over.
"We've seen it before and we're seeing it again. Ugly phone calls, misleading mail and TV ads, careless, outrageous comments."
As a crowd of thousands at a Democratic rally in Orlando, Florida later in the day chanted "Yes we can!" Clinton told supporters: "And not only that, yes we will!"
"We cannot falter, we cannot stop, we cannot take a single vote for granted."
She blamed recent economic turmoil in the US on the policies espoused by George Bush, the US president, and McCain.
"Now is the time to ... close the book on eight years of failed Republican leadership," Clinton said.
Obama, speaking in Orlando, held only praised for Clinton, who he faced-off against in a heated Democratic primary contest.
"I've admired her as a leader, I've learned from her as a candidate," he said. "I'm proud to call her my friend."
'Spread the wealth'
A daily tracking survey released on Sunday by Gallup showed Obama leading McCain nationally by 10 percentage points, at 52 to 42, after declining to as little six points last week.
The Arizona senator is focusing on tax and economic issues as the campaign enters its final 15 days.
Al Jazeera's Rosiland Jordan, reporting from St Charles, said: "McCain says he's not conceding the race to Barack Obama."
He told supporters in Missouri that "nothing is inevitable" and he could still beat Obama.
"I am an American and I choose to fight! Don't give up your courage."
The McCain campaign has focussed on a comment Obama made at rally about two weeks ago.
"We finally learned his goal - [Obama] wants to 'spread the wealth around!'" McCain said in St Charles, Missouri.
But the endorsement made by Powell on Sunday has hurt the McCain campaign as he had been a very senior figure in the early years of the Republican Bush administration, Jordan said.
Voters in every state began to cast ballots under rules that allow people to register their votes ahead of the main November 4 election day.
The results will not be released until the full poll, but the numbers of those who have voted suggests the Democrats have been successful in their campaigning.
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.
Obama's campaign has been targeting early voters.
"We are trying to expand the electorate and expand the process," David Plouffe, Obama's campaign manager.
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 November 4.
Obama's campaign announced the candidate would be leaving the campaign trail for two days and cancelling some scheduled events in order see his ailing grandmother in Hawaii.
Robert Gibbs, an Obama aide, said: "Recently his grandmother has become ill and in the last few weeks her health has deteriorated to the point where her situation is very serious."
Obama intends to fly to Hawaii on Thursday and return to the campaign trail on Saturday, Gibbs said.