"You have put us within reach of the Democratic nomination for president of the United States. We have returned to Iowa with a majority of delegates elected by the American people," he told a crowd of supporters here. While Clinton's campaign continued to make a case that she could prevail, Obama was poised to use the results from Democratic contests in Kentucky and Oregon to move into a new phase of the campaign in which he will face different challenges. Those include bringing Clinton's supporters into his camp; winning over elements of the Democratic coalition like working-class whites, Hispanics and Jews; and fending off attacks from Senator John McCain, the presumptive Republican nominee, especially on national security.
Obama's obstacles were underlined by what was a lopsided defeat in Kentucky, where just half of the Democratic voters said in exit polls that they would back Obama in the general election this fall.
But under the rules used by Democrats, Obama picked up additional delegates even in defeat, moving him closer to securing a majority of the delegates up for grabs in primaries and caucuses.
Obama's campaign expected to exceed that threshold, which it has portrayed as the proper yardstick for judging the will of Democratic voters, by the time the results are in from Oregon, where the polls close at 11 p.m. Eastern time.
Even as Obama moved closer to making history as the first black presidential nominee, he stopped short of declaring victory in the Democratic race, part of a carefully calibrated effort in the remaining weeks of the contest to avoid appearing disrespectful to Clinton and alienating her supporters.
In his speech here on Tuesday, Obama sang the praises of his opponent, noting that "no matter how this primary ends, Senator Clinton has shattered myths and broken barriers." Clinton has made clear that she has no intention of stepping aside before the Democratic voting ends on June 3.
Going into Tuesday, Obama had 1,915 of the 2,026 pledged delegates and super delegates needed to claim the nomination, according to a count and projection by The New York Times.
His campaign estimated that if he simply held his own in the remaining contests, he would need only 25 more votes from super delegates, the elected Democrats and party leaders who are delegates without having to be selected in a primary.
There are 221 undeclared super delegates left; Obama has been rolling out endorsements on a daily basis.
Obama is scheduled to spend Wednesday through Friday in Florida, focusing specifically on the corridor between Tampa and Orlando, a region bustling with swing voters.
In the next month, he will head to Michigan, Missouri, Pennsylvania and Ohio, even as he continues to spend time where the remaining three contests will be held.
Since 1972, when exit polls first began, no Democratic presidential candidate has won a majority of white voters. The closest division was in 1992, a three-way contest when 39 percent of whites voted for Bill Clinton and 40 percent voted for the first President Bush. In 2004, President Bush defeated John Kerry among whites by 58 percent to 41 percent.