Obama lavished praise on Clinton, his rival in a race unlike any other, and accused Republican John McCain of a campaign run by lobbyists.
"You are Democrats who are tired of being divided, Republicans who no longer recognize the party that runs Washington, independents who are hungry for change," he said, speaking to a crowd on the grounds of the Iowa Capitol in Des Moines as well as the millions around the country who will elect the nation's 44th president in November.
Clinton countered with a lopsided win in Kentucky, a victory with scant political value in a race moving inexorably in Obama's direction.
The former first lady vowed to remain in the race, telling supporters, "I'm more than determined than ever to see that every vote is cast and every ballot is counted."
But in a sign of confidence on the front-runner's part, party officials said discussions were under way to send Paul Tewes, a top Obama campaign aide, to the Democratic National Committee to oversee operations for the fall campaign.
And in a fresh indication that their race was coming to an end, Clinton and Obama praised one another and pledged a united party for the general election.
"While we continue to go toe-to-toe for this nomination, we do see eye-to-eye when it comes to uniting our party to elect a Democratic president this fall," said Clinton, whose supporters Obama will need if he is to end eight years of Republican rule in the White House.
Clinton won at least 47 delegates in the two states and Obama won at least 32, according to an analysis of election returns by The Associated Press. All the Kentucky delegates were awarded, but there were still 24 to be allocated in Oregon, and Obama was in line for many of them.
He had 1,949 delegates overall, out of 2026 needed for the nomination. Clinton had 1,769 according the latest tally by the AP.
Obama's total includes more than a majority of the delegates picked in the 56 primaries and caucuses on the calendar, a group that excludes nearly 800 superdelegates, the party leaders who hold the balance of power at the convention.
With about 50 percent of the votes counted in Oregon's unique mail-in primary, Obama was gaining a 58 percent share to 42 percent for Clinton.
The former first lady's victory in Kentucky was bigger yet - 65 percent to 30 percent - and the exit polls underscored once more the work Obama has ahead if he is to win over her voters.
Almost nine in 10 ballots were cast by whites, and the former first lady was winning their support overwhelmingly. She defeated him among voters of all age groups and incomes, the college educated and non-college educated, self-described liberals, moderates and conservatives.
"We have had our disagreements during this campaign, but we all admire her courage, her commitment and her perseverance," Obama said of his rival and partner in a marathon race through the primaries. "No matter how this primary ends, Senator Clinton has shattered myths and broken barriers and changed the America in which my daughters and yours will come of age."
As for McCain, he said he would leave it up to the Arizona senator "to explain whether his policies and positions represent long-held convictions or Washington calculations, but the one thing they don't represent is change."
McCain's spokesman countered quickly.
"This election is fundamentally about who Americans can trust to secure peace and prosperity for the next generation of Americans. Without a doubt, Barack Obama is a talented political orator, but his naive plans for unconditional summits with rogue leaders and support for big tax hikes on hardworking families expose his bad judgment that Americans can ill-afford in our next president," said Tucker Bounds in a statement.
In the fundraising chase, Obama reported cash on hand of $46.5 million, all of which can be used for the general election. Unless he takes federal funds, he is permitted to raise as much as he can.
Unlike Obama, McCain is expected to take federal funds, which total about $85 million and bar him from raising other donations for his campaign's use.
"We still have work to do to in the remaining states, where we will compete for every delegate available," Obama said in an e-mail sent to supporters. "But tonight, I want to thank you for everything you have done to take us this far - farther than anyone predicted, expected or even believed possible."
Both candidates paused during the day to express best wishes to Sen. Edward M. Kennedy, the Massachusetts Democrat suffering from a brain tumor.
"So many of us here have benefited in some way or another because of the battles he's waged, and some of us are here because of them," Obama said.
Said Clinton: "As a lifelong champion for social justice and equality, his work has made the path easier for me, for Senator Obama and for countless others. He's been with us for our fights and we're now with him in his."
The Clinton campaign expressed irritation at Obama's decision to return to Iowa and mark his success in amassing a majority of delegates won in primaries and caucuses.
But he paid no attention. "The question then becomes how do we complete the nomination process so that we have the majority of the total number of delegates, including superdelegates, to be able to say this thing's over," Obama told The Associated Press in an interview.
Clinton looked for a consolation for the strongest presidential campaign of any woman in history. She hoped to finish with more votes than her rival in all the contests combined, including Florida and Michigan, two states that were stripped of their delegates by the national party for moving their primary dates too early. A Democratic convention committee is to meet on May 31 in Washington to decide how - and whether - to seat delegates from the two states.
Not counting the results in Kentucky and Oregon, Obama was ahead of Clinton by slightly more than 618,000 votes out of 32.2 million cast in primaries and caucuses where both candidates competed.
The numbers do not include Iowa, Maine, or Nevada caucuses, nor do they count - as Clinton does in her totals - Florida and Michigan.
The only primaries remaining are Puerto Rico, on June 1, followed two days later by South Dakota and Montana.
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David Espo reported from Washington. Brendan Farrington in
Florida contributed to this report.
