Polls show the six closest states are Florida, Indiana, Missouri, North Carolina, Nevada and Ohio. All were won by Bush and made competitive by Obama's record-shattering fundraising. The campaigns also are running aggressive ground games elsewhere, including Iowa, New Mexico, Pennsylvania, New Hampshire, Colorado and Virginia.
All that's left now for the candidates is make sure people vote Tuesday - if they haven't already.
Indeed, Election Day is becoming a misnomer. About 27 million absentee and early votes were cast in 30 states as of Saturday night, more than ever. Democrats outnumbered Republicans in pre-Election Day voting in key states.
That has Democrats - and even some Republicans - privately questioning whether McCain can overtake Obama, even if GOP loyalists turn out in droves on Tuesday. Obama may already have too big of a head start in critical states like Nevada and Iowa, which Bush won four years ago.
As the campaign closes, voters were being inundated with a crush of television ads and automated phone calls.
In a new TV ad, Obama highlighted Vice President Dick Cheney's support for McCain. The ad features Cheney, an extremely unpopular figure among the general public, at an event Saturday in Wyoming, saying: "I'm delighted to support John McCain."
Not to be outdone, the Republican National Committee rolled out battleground phone calls that include Hillary Rodham Clinton's criticism of Obama during the Democratic primary. She is heard saying: "In the White House, there is no time for speeches and on-the-job training. Sen. McCain will bring a lifetime of experience to the campaign, and Sen. Obama will bring a speech that he gave in 2002." A Clinton spokeswoman said she disapproves of the ad.
The Pennsylvania GOP also unveiled a TV ad featuring Obama's former pastor, the Rev. Jeremiah Wright, declaring "God damn America!" in a sermon.
Obama and McCain campaigned on each other's turf Sunday. Obama was in Ohio, a bellwether state Bush won four years ago and where polls show Obama tied or winning. McCain visited Pennsylvania and New Hampshire, states won by Democrat John Kerry in 2004. He trails in both.
McCain and the RNC dramatically ramped up their spending in the campaign's final days and now are matching Obama ad for ad, if not exceeding him, in key battleground markets in states such as Florida, Ohio, North Carolina, Virginia and Pennsylvania.
After months of planning, the Republican Party launched the last stage of its vaunted "72-hour program," when volunteers descend on competitive states for the final stretch. Democrats unleashed their "persuasion army" of backers scouring their own backyards to encourage people to back Obama in the campaign's waning hours.
Obama's campaign reported that Saturday was its largest volunteer day, with more volunteers showing up to work the phones and walk neighborhood precincts than ever before in the campaign. Said Obama spokesman Bill Burton, "Our volunteers are completely engaged."
McCain's crew says theirs are, too.
"There's no doubt that we've got an uphill battle," said Rich Beeson, the RNC's political director. But, "We still have a lot of voters that we can and will turn out."
The RNC reported making 5.4 million voter contacts last week, compared with 1.9 million in the same week in 2004, and it says it's volume has steadily increased since October began. Overall, it says 26 million voters have been contacted by volunteers over four months.
On Saturday alone, the RNC says an estimated 3 million voters were contacted by phone or in person, and it saw so many volunteers show up to help that in at least one state, Colorado, the party ran out of canvassing packets. Some 180,000 were gone by midday Saturday; more were printed.
McCain planned visits to media markets that hit battlegrounds Florida, Virginia, Indiana, New Mexico, and Nevada on Monday. A repeat trip to Pennsylvania also was slated before McCain returns home to Arizona.
Obama planned visits to Florida, North Carolina and Virginia on Monday and a quick stop in Indiana Tuesday morning. He told reporters he would hold a news conference on Wednesday. Later, Obama spokeswoman Linda Douglass walked back Obama's plans, saying he'll meet the press before the end of the week, but "don't count on Wednesday."