The US presidential rivals are heading back to the campaign trail after a heated final televised debate.
John McCain turned in an aggressive performance but opinion polls on the event suggested he had failed to land a telling blow on Barack Obama.
Analysts say the Republican Mr McCain will now spend some time focusing on shoring up stronghold states.
Frontrunner Mr Obama advised against complacency, recalling his surprise defeat in the New Hampshire primary.
On Thursday night the two candidates will again share a stage - giving speeches at a traditional political dinner in Manhattan in honour of former New York Governor Al Smith.
Mr Obama will now be targeting states he previously considered to be Republican territory, commentators say.
He is heading out to visit Virginia and Missouri in the next few days.
Sources in the Democratic campaign say he will advertise in West Virginia, which George W Bush won four years ago and which Mr Obama lost to Democratic rival Hillary Clinton in the primary.
The sources say the campaign may also fund attempts to win over states such as Kentucky, North Dakota and Georgia.
Mr McCain will visit the swing state of Pennsylvania but sources say he will also have to go to more favourable Republican territories such as Virginia, Colorado and Florida.
The Republican National Committee (RNC) has said it will halt advertising in Maine and Wisconsin. The move suggests strategists are doubtful about their candidate's chances there, analysts say.
The McCain campaign will continue to run its own ads in the states. Mr McCain was in Wisconsin last week, and running mate Sarah Palin is expected to visit Maine on Thursday.
'I'm not Bush'
On Thursday morning, Mr Obama said there were now 19 days to go but that would be a beginning not an end, as the amount of work for the next president would be "extraordinary".
He warned against complacency, referring to his defeat by Mrs Clinton in the New Hampshire primary.
"I've been in these positions before where we were favoured and the press starts getting carried away, and we end up getting spanked," Mr Obama said.
In Wednesday's 90-minute TV debate at Hofstra University in Long Island, New York, analysts said Mr McCain had turned in his best performance.
He went on the attack immediately, calling Mr Obama's tax plans class warfare and deriding efforts to link him to President George W Bush.
"Senator Obama, I am not President Bush. If you wanted to run against President Bush, you should have run four years ago," Mr McCain said.
He also referred to Mr Obama's association with Bill Ayers, once a member of a US group that waged a violent campaign against the Vietnam War.
However, Mr Obama appeared unruffled by the attacks, saying Mr McCain had been "a vigorous supporter of President Bush".
He added: "Mr Ayers is not involved in my campaign. He has never been involved in this campaign. And he will not advise me in the White House."
The Illinois senator said that 100% of Mr McCain's political adverts had been negative and that voters were more interested in how the candidates planned to fix the economy.
Immediate voter polls found Mr Obama came out on top.
A CNN poll of people watching Wednesday's debate said Mr Obama won by 58% to 31%, while a CBS survey found the Democrat the winner by 53% to 22%.
A poll of undecided independent voters by US network Fox also suggested Mr Obama was the victor.
One interested viewer was Joe Wurzelbacher.
He shot to prominence in the campaign when he met Barack Obama and expressed concern about what the senator's tax plans might mean for his plumbing business.
After the debate "Joe the plumber" from Ohio said he was "floored" by the impact he had made.