Obama paid tribute to his rival Hillary Clinton, who coasted to an overwhelming victory in the Kentucky primary on Tuesday.
With votes counted from 97 per cent of the Kentucky precincts, Clinton had won 65 per cent support compared to 30 per cent for Obama.
Obama made his remarks before voting closed in Oregon and Al Jazeera's US media partner NBC has now projected that he will win the state, with 60 per cent of the vote versus Clinton's 40 per cent.
Obama also sought unity within the Democratic party, and with Clinton's supporters.
"Some may see the millions and millions of votes cast for each of us as evidence our parity is divided, but I see it as proof that we have never been more energised and united in our desire to take this country in a new direction."
But the tenor of his speech left little doubt that he has put the lengthy and hard fought contest against Clinton behind him and is looking ahead to the battle against John McCain, the all but certain Republican candidate.
Clinton not quitting
In a speech in Kentucky following her victory, however, Clinton vowed to battle on to end of the nominating contests on June 3.
"Some have said this was over, but not allowing everyone to vote would be a mistake," she said.
Her campaign director, Olivia Ann Morris Fuchs, told Al Jazeera she was certain her candidate would win and denied Clinton's continued candidacy was hurting the Democrats.
Clinton's argument is based on her victories in Florida and Michigan.
The Democratic party stripped those states of their delegates because they violated party rules and held their primary polls early.
The Clinton camp has begun to include numbers from the two states in its calculation of how many delegates are needed to secure the nomination, saying that the total should be 2210.
But the party and Obama say 2026 delegates are needed.
The party is expected to decide the fate of Florida and Michigan's delegates on May 31.
Clinton says all 50 states should have a chance to cast ballots before the nominee is decided, with the last contest scheduled for June 3.
Clinton also claims she has the best chance of defeating McCain because she won in swing states such as Ohio, pivotal to a presidential election victory in November.
"We're confident of [Clinton] being the nominee. She's ahead in the total popular vote and she has as many delegates as she needs," Fuchs said.
"She's not hurting our party - we believe in every voice being heard and every vote counting."
Fuchs said Clinton would become the Democratic candidate for the presidency as only she could win in the key swing states such as Pennsylvania and Ohio.
But many in Obama's campaign are already looking ahead to the presidential election in November.
"A clear majority of elected delegates will send an unmistakable message - the people have spoken and they are ready for change," David Plouffe, Obama's campaign manager said in a message to supporters.
The only primaries remaining are Puerto Rico, on June 1, followed two days later by South Dakota and Montana.
'Low road' warning
Obama warned of a tough campaign ahead against the Republicans and McCain.
"They will play on our fears and out doubts and our divisions to distract us from what matters to you," he said on Tuesday.
|Clinton vowed to press on after |
her Kentucky win [AFP]
"Well, they can take the low road if they want, but it will not lead this country to a better place. And it will not work in this election. It won't work because you won't let it," he told 6,000 supporters at an outdoor rally near the Iowa Statehouse.
Obama scheduled the rally in Iowa instead of in one of the states holding primaries on Tuesday as a way of "coming full circle" and launching the general election campaign in a place that is likely to be competitive come November.
"The same question that first led us to Iowa 15 months ago is the one that has brought us back here tonight," Obama said on Tuesday.
"The question of whether this country, at this moment, will keep doing what we've been doing for four more years or whether we will take that different path.
"It is more of the same versus change," he said.
While Obama will still not have enough of the 2,026 nominating delegates overall needed to win the nomination after Tuesday's polls, he is likely to secure enough pledged delegates to make it almost impossible for Clinton to overcome his lead, analysts say.
Obama also leads in the so-called "superdelegate" race - senior Democratic party members who vote on the nomination at the party's August convention.
On Wednesday, both candidates are set to campaign in Florida ahead of the key Democratic party decision on its delegates on May 31.