Arizona, California, Connecticut, Delaware, Florida, Illinois, Maryland, Missouri, New Hampshire, New Jersey, New York, Ohio, Oklahoma, Pennsylvania, Rhode Island, South Carolina, Texas, Vermont, Virginia, Washington state, Wisconsin
Mitt Romney 11 states, 251 delegates
Alaska, Colorado, Maine, Massachusetts, Michigan, Minnesota, Montana, Nevada, North Dakota, Wyoming, Utah
1,191 delegates needed for nomination. Source: AP (includes all kinds of delegates)
Speaking in Houston, Mr Obama said the change he wanted would be hard to achieve.
"It is going to require more than rousing speeches... It is going to require something more, because the problem that we face in America today is not the lack of good ideas. It's that Washington has become a place where good ideas go to die."
Republican frontrunner John McCain, who is now virtually assured of his party's nomination, also appeared to attack Mr Obama.
"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," he said.
Women and youth
In the Wisconsin and Hawaii contests, Mr Obama amassed at least 55 of the delegates who will officially nominate the Democratic candidate - compared to Mrs Clinton's 33.
He now boasts a total of 1,335 of the delegates to his rival's 1,251. Six delegates are still to be allocated.
It will take 2,025 delegates to secure the Democratic nomination at the party's national convention this summer.
The Illinois senator was reported to have gained almost equal support from white women, and to have polled well from working-class Democrats - both groups that have usually supported Mrs Clinton.
Mr Obama also took the youth vote and six out of 10 voters who described themselves as independent, according to exit polls for ABC.
The Ohio and Texas primaries will be held on 4 March, together with the smaller New England states of Vermont and Rhode Island.
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