McCain, meanwhile, moved closer to getting the Republican presidential nomination on Tuesday, winning in Wisconsin and gaining a big lead in early results from Washington state.
"Thank you Wisconsin for bringing us to the point where even a superstitious naval aviator can claim with confidence and humility that I will be our party's nominee for president," McCain, a former Navy fighter pilot and Vietnam prisoner of war, told supporters in Columbus, Ohio.
Obama targetted
McCain took direct aim at Obama in his victory remarks.
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"I will fight every moment of every day in this campaign to make sure that Americans are not deceived by an eloquent but empty call for change," the Republican nominee-in-waiting said in a thinly veiled attack on Obama.
His words seemed to echo Clinton's, who all but conceded Wisconsin even before the release of results.
Addressing supporters, Clinton never mentioned her Wisconsin loss and instead attacked Obama's oratory as empty and meaningless.
The primary campaign "is about picking a president who relies not just on words but on work - on hard work to get America back to work", she told a Youngstown, Ohio, rally on Tuesday night, adding that the "best words in the world are not enough" unless they are matched with action.
Tight race
Obama began the evening with eight straight primary and caucus victories, a run that has propelled him past Clinton in the overall delegate race and enabled him to chip away at her advantage among elected officials within the party who will have convention votes as "super delegates".
Clinton, who was hoping to blunt some of Obama's momentum, will have to win next month in Ohio and Texas to salvage her presidential campaign.
Democrats and Republicans on Tuesday also voted in Hawaii, where Obama was born and is a heavy favourite.