By John Whitesides, Political Correspondent
In their hard-fought duel for the Democratic nomination, Obama won 11 states and Clinton took eight on the biggest day of U.S. presidential voting ahead of November's election. Three Democratic contests were still undecided in the early hours of Wednesday.
McCain had had hoped to nail down the nomination with a big night and his wins included several big Northeastern states, but rival Mitt Romney took six states and Mike Huckabee won five.
The biggest prize of the night was California, which offers the country's biggest haul of delegates to party conventions that choose the parties' presidential candidates for the November 4 election to succeed President George W. Bush.
The mixed outcome in the coast-to-coast voting, with all contenders in both parties scoring at least five wins, appeared certain to prolong the hard-fought nominating races in both parties. More contests in a half-dozen states are slated in the coming week.
National exit polls showed more than half of Democratic voters ranked the ability to bring change as the top attribute for a candidate. Nearly one-quarter of Democrats voting in the party's 22 contests ranked experience, Clinton's selling card, as the most important attribute.
More than half the total delegates to the Democratic convention in August and about 40 percent of the delegates to the Republican convention in September will be apportioned in Tuesday's voting.
Economic worries -- plunging housing values, rising energy and food prices, jittery financial markets and new data showing a big contraction in the service sector -- eclipsed the Iraq war as voters' top concern in both parties, exit polls showed.
McCain won in Connecticut, New Jersey, Delaware, New York, Oklahoma, Arizona, Illinois and Missouri.
"It's not all done tonight. We're going to keep on battling," Romney said in Boston.
"A lot of people have been trying to say this is a two-man race," Huckabee told supporters in Little Rock, Arkansas. "Well, you know what, it is and we're in it."
"I look forward to continuing our campaign and our debate about how to leave this country better off for the next generation," Clinton told supporters in New York, congratulating Obama on his wins.
She and Obama had split the first four significant nominating contests in January and spent heavily on advertising across the country.
In contrast, many of the 21 Republican contests are winner-take-all when awarding delegates, meaning a strong day by McCain could give him a commanding lead.
(Additional reporting by Jeff Mason, Claudia Parsons, Steve Holland, Ellen Wulfhorst, Andy Sullivan; editing by Frances Kerry)
