Article published on the 2008-05-21 Latest update 2008-05-22 18:28 TU
Obama told a 7,000-strong crowd in Iowa that he now had "a majority of the delegates elected by the American people".
The nearly 800 unelected "super-delegates" remain an unknown quantity in the final choice.
Apparently confident of victory, he praised Clinton and lashed out at Republican nominee John McCain.
"No matter how this primary season ends," he said, "Senator Clinton has shattered myths and broken barriers and changed the America in which my daughters and yours will have come of age."
He declared that Bush's Iraq policy "which asks everything of our troops and nothing of Iraqi politicians" is also McCain's and added that the Republican would continue a diplomatic policy which has "left this country more isolated and less secure than at any time in recent history".
In Kentucky Clinton promised to "keep fighting until we have a nominee, whoever she may be".
Obama's campaign says that it has raised 31 million dollars (20 million euros) since April, while the Clinton side says it has raised 22 million dollars (14 million euros) in the same time.
Both candidates paid tribute to Senator Edward Kennedy, who has been diagnosed as having a malignant brain tumour.