Obama seeks to boost new voters
Mr McCain is preparing for Tuesday's debate while Mr Obama campaigns
US Democratic presidential hopeful Barack Obama is holding a rally in the state of Virginia, ahead of a Monday deadline for voters to register there.
The Obama campaign has also organised shows by Bruce Springsteen and others in Michigan, Pennsylvania and Ohio in a bid to promote new voter registration.
Republican rival John McCain will spend the weekend in Arizona preparing for Tuesday's second presidential debate.
His running mate, Alaska Governor Sarah Palin, is to campaign in California.
Monday is the last day on which new voters can register in more than a dozen states, including Pennsylvania, Michigan, Ohio, Indiana, Florida and Virginia.
Singers Bruce Springsteen and Jay-Z are due to play at events in Philadelphia and Detroit, as well as Ohio State University, where the Obama campaign may hope to sign up first-time voters.
Voter turnout could be key in deciding the outcome of the 4 November presidential election.
The Obama campaign has turned its focus to Mr McCain's proposals on healthcare this weekend, with new adverts playing on TV and radio and leaflets sent to homes in every battleground state.
Mr Obama is also expected to criticise his rival's policy at a rally in Newport News, Virginia, on Saturday.
Record numbers watched Joe Biden and Sarah Palin in the VP debate
According to remarks released in advance, he will tell voters that Mr McCain's plan will encourage younger, healthier workers to buy cheaper health insurance outside the workplace, causing many employers to abandon their schemes.
He will say: "Study after study has shown, that under the McCain plan, at least 20 million Americans will lose the insurance they rely on from their workplace."
A spokesman for the Republican National Committee said Mr Obama was "lying" about his rival's proposals.
"Obama's plan only offers more government, while McCain's plan offers more choices," the Associated Press quotes spokesman Alex Conant as saying.
Mr McCain has proposed tax credits to help more people pay for health insurance, while Mr Obama wants to bring about universal coverage by providing subsidies to make it more affordable.
Opinion polls suggest healthcare is an important issue for voters.
Meanwhile, viewing figures show a record 69.9m people tuned in to watch Mrs Palin take part in Thursday's televised vice-presidential debate with Democratic nominee Joe Biden.
That number eclipsed the mark set in 1984, when 56.7m people watched Geraldine Ferraro, the only previous female US vice-presidential candidate, go head-to-head with George Bush Senior.