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)
In terms of delegates captured, Mrs Clinton was only marginally ahead, under the Democrats' system of proportional distribution.
Both are still well short of the 2,025 delegates needed to secure the party's nomination.
Overall, voters were choosing 42% of delegates on Super Tuesday.
The BBC's North America editor Justin Webb points out that Mrs Clinton won in important states such as New York, New Jersey and California, and that her support there looks like the solid foundation of a Democratic Party victory in November's presidential poll.
In contrast, he says, Mr Obama's wins were in states such as Georgia and North Dakota, which will not be important Democratic targets come November.
Mr Obama did well but there are still questions about the long-term viability of his campaign, our editor says.
Mr McCain called for unity in the Republican party after a series of poll victories that correspondents say seriously wounded his main rival, Mitt Romney.
Mr McCain enjoyed wins in the big states of California and New York, as well as Illinois, Oklahoma, New Jersey, Missouri, Connecticut, Delaware, and his home state of Arizona.
WHAT IS SUPER TUESDAY?
24 states holding simultaneous contests to help decide the Democratic and Republican presidential nominations
About 40% of each party's delegates - who will choose the candidate - are up for grabs
Key states electing large numbers of delegates include California, New York and Illinois
But five wins for former Baptist minister Mike Huckabee, who was polling a distant third but is popular with evangelical Christians, backed up the widely held view that Mr McCain lacks support from conservatives in his own party.
Some key conservative figures have refused to vote for Mr McCain in the presidential election if he wins the nomination.
Speaking on Wednesday, Mr McCain said: ""I do hope that at some point we would just calm down a little bit and see if there are areas that we can agree on for the good of the party and for the good of the country."
Both Mr McCain's rivals, Mr Huckabee and Mr Romney, have vowed to fight on.
Our correspondent says the presidential race is living up to its billing as the most unpredictable in living memory.
He says a Republican nominee may emerge earlier on, but one who lacks party support, to face two Democrats who both have enthusiastic party backing but could spend many more months fighting each other.
But correspondents say the Democratic contenders will also increasingly be putting pressure on so-called super-delegates, party members who are able to decide for themselves who to back at the convention, rather than being obliged to back whoever their state's voters support.