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)
Maggie Williams, who was the New York senator's chief of staff when her husband was serving as US president, will take over from Patti Solis Doyle, who has decided to step down.
Campaign aides said Ms Doyle's decision was not urged upon her by Mrs Clinton or any senior members of her team, the Associated Press reports.
Mr Obama and Mrs Clinton now head into a series of contests on Tuesday: Virginia, Maryland and the District of Columbia.
Across Maine on Sunday, Democrats in 420 cities and towns braved harsh winds and snowfall to turn out in heavy numbers for the state-wide caucuses.
They were deciding how the state's 24 delegates will be allotted at August's Democratic party national convention.
Mr Obama is now projected to pick up 15 delegates, with the remaining nine going to Mrs Clinton.
On the Republican side, President Bush has described the party's front-runner, John McCain, as a true conservative.
Tuesday: Maryland, Virginia and Washington DC (multi-party)
The presidential vote of confidence comes despite continuing criticisms from leading party members who have questioned the Arizona senator's conservative credentials.
Although Mr McCain won primary polling in Washington state on Saturday, correspondents say the Arizona senator still has some work to do to unite his party, as Mike Huckabee won in Kansas and Louisiana.
Mr Huckabee and third-placed Ron Paul have been coming under pressure to step aside for the sake of party unity.
But Mr Huckabee said on Saturday that he had no intention of pulling out.
Mr McCain has a wide lead with 719 delegates to Mr Huckabee's 234 and Mr Paul's 14.
Mr Romney's suspended campaign still has 298 delegates.
On the Democratic side, Mr Obama and Mrs Clinton are facing the prospect of a long drawn-out battle after neither was able to deliver a knock-out blow in Super Tuesday's 22 state contests.
Mrs Clinton and Mr Obama are practically level, each halfway to winning the 2,025 delegates needed to secure victory at the Democratic party convention in August.