Democratic candidate Barack Obama still holds a poll lead, but one new survey suggests his lead over Republican John McCain is narrowing slightly.
Mr McCain holds rallies in Virginia and Pennsylvania before appearing on TV show Saturday Night Live.
Mr Obama campaigns in Nevada, Colorado and bellwether state Missouri.
Both men are visiting states seen as crucial to their chances of winning Tuesday's election.
Analysts say Mr McCain needs to win in Pennsylvania - where he is behind in state polls - to have a chance. Polls in Virginia, which voted Republican in 2004, show Mr Obama has pulled ahead of his rival there.
Mr Obama also leads Mr McCain in Nevada and Colorado, both of which voted for George W Bush four years ago.
Missouri is seen as a vital state to win because of its record of backing the eventual winner in almost every election since 1904. Mr Obama leads in Missouri by a very narrow margin, polls show.
Mr Obama is extending his campaign advertising into traditionally Republican territory over the weekend, running advertisements in Arizona, his rival's home state, as well as Georgia and North Dakota.
As the candidates focus on battleground states last-minute preparations are being made for Tuesday's vote, the BBC's North America editor Justin Webb reports.
Polling officials are expecting some 130 million Americans to vote - a turnout which would prove higher than in any election since 1960.
Security is of most concern in Chicago, Mr Obama's base, where up to one million people are expected to turn out in the city centre for what they hope will be a White House victory for the Illinois senator.
But Mr McCain is remaining defiant, playing on his status as the underdog and telling supporters on Friday night that he would still make a late run to victory.
He was boosted by the support of California Governor Arnold Schwarzenegger, who gave a rousing speech in the swing state of Ohio.
The ex-film star said Mr McCain was a real "action hero" who had spent more time as a prisoner of war than Barack Obama had served in the US Senate.
"We're closing, my friends, and we're going to win in Ohio," Mr McCain told the Ohio crowd.
"We're a few points down but we're coming back and we're coming back strong."
In a new poll Reuters/Zogby poll released on Saturday Mr Obama's lead was down to 5%, at 49% to Mr McCain's 45%. The gap narrowed from 7% in the previous survey.
Mr Obama warned supporters to expect a hard fight until the very end of the campaign.
"We are four days away from changing the United States of America," he said
Our correspondent says that while the McCain camp claim their candidate is just four points behind in the national polls, his problem is that in many states that he has to win, he is behind.
No Republican has ever been elected without winning Ohio, Justin Webb adds, and John McCain appears to be five points adrift there at the moment.