Delivering what his campaign billed as his "closing argument," Obama told a crowd in Canton, Ohio, that "there's so much at stake" in the week ahead.
"We cannot let up for one day, one minute, or one second in this last week. Not now," he said.
The Democratic candidate, who has a sizeable lead in national polls, warned against acting as if the election is already over.
"Don't think for a minute that power concedes. We have a lot of work to do. We have to work like our future depends on it in this last week, because it does depend on it this week," he said.
Obama told voters it was up to them to "give this country the change we need," as he tried to make the case that Sen. John McCain is too similar to President Bush to bring about that change.
"Sen. McCain says that we can't spend the next four years waiting for our luck to change, but you understand that the biggest gamble we can take is to embrace the same old Bush-McCain policies that have failed us for the last eight years," Obama said.
The McCain campaign blasted Obama's remarks as "an argument for closing down opportunity."
"It's fitting that during Barack Obama's 'closing argument,' he was unable to support his rhetoric with a single accomplishment ... Barack Obama's 'closing argument' is the same old argument in favor of job-killing tax increases and massive new spending," spokesman Tucker Bounds said.
McCain on Monday sought to assure voters that his administration would be far different from the Bush administration.
"This is the fundamental difference between Sen. Obama and me. We both disagree with President Bush on economic policy. The difference is he thinks taxes have been too low, and I think that spending has been too high," McCain said at a rally in Dayton, Ohio.
McCain tried to paint Obama as a candidate who "believes in redistributing wealth, not in policies that grow our economy and create jobs."
The Arizona senator pointed to a 2001 radio interview in which Obama said that one of the failures of the civil rights movement was that "the Supreme Court never entered into the issues of redistribution of wealth, and sort of more basic issues of political and economic justice in this society."
McCain said that in the Obama campaign, change means "taking your money and giving it to someone else."
The Obama campaign called McCain's attack on the interview a "fake news controversy drummed up by the all too common alliance of Fox News, the Drudge Report and John McCain, who apparently decided to close out his campaign with the same false, desperate attacks that have failed for months."
McCain on Monday also warned voters of what he sees as the dangers of a government controlled by Democrats.
Democrats in the Senate are hoping to win 60 seats -- enough to secure a filibuster-proof majority in the Senate.
"The answer to a slowing economy is not higher taxes, but that is exactly what is going to happen when the Democrats have total control in Washington. Can you imagine an Obama, [Nancy] Pelosi, [Harry] Reid combination? We can't let that happen," he said.
With just eight days left for each side to make its case, Obama and McCain were both focusing on battleground Ohio, where 20 electoral votes are at stake.
Obama and McCain both planned to campaign in Pennsylvania following their Ohio events.
Obama leads McCain by 4 points in Ohio, 50 percent to 46 percent, according to CNN's average of polls there.
The Democrat also has the lead in Pennsylvania, up 51-41 percent, according to CNN's poll of the polls there.
Pennsylvania Gov. Ed Rendell recently sent two separate memos to the Obama campaign requesting that the Democratic candidate as well as Hillary and Bill Clinton return to his state.
Obama's last visit to the state was on October 11.
Rendell said the McCain campaign was clearly making a push to win Pennsylvania, given the recent visits by the Arizona senator, his wife and his running mate, Alaska Gov. Sarah Palin. According to Rendell, there is also worry among Democrats that the McCain campaign has successfully raised the enthusiasm level among Republicans in the state.
Palin had a series of rallies scheduled Monday in Virginia, where Republicans now find themselves playing defense.
CNN's poll of polls in Virginia shows Obama leading McCain, 51-44 percent. The state hasn't voted for a Democratic president in more than four decades.
Sen. Joe Biden, the Democratic vice presidential nominee, had rallies planned in North Carolina and Florida, two states that President Bush won in the past two presidential elections.
Obama leads by 2 points in Florida, 48-46 percent, according to the most recent average of polls there.
In North Carolina, a recent CNN/Time/Opinion Research Corp. poll indicated Obama had a 4-point lead over McCain, 51 percent to 47 percent
The poll was conducted October 19 through October 21 and has a margin of error of plus or minus 4 percentage points.