Democrat Barack Obama and rival Hillary Clinton battled to a draw on "Super Tuesday" and John McCain seized command of the Republican race in presidential nominating contests in 24 US states.
In their hard-fought duel for the Democratic nomination, Obama won 12 states and Clinton took eight. Clinton's wins included the key prizes of California and New York on the biggest day of US presidential voting ahead of November's election.
"There is one thing on this February night that we do not need the final results to know: Our time has come," Obama told cheering supporters in Chicago. "Our movement is real, and change is coming to America."
McCain won nine states, including key victories in California and big Northeastern states to take a daunting lead in the Republican race. He captured a huge haul of delegates who select the party's presidential nominee, in part because many of the 21 Republican contests were winner-take-all.
Republican rivals Mitt Romney and Mike Huckabee kept their hopes alive and vowed to fight on. Romney won six states and Huckabee won five.
"Tonight, I think we must get used to the idea that we are the Republican Party frontrunner for the nomination," McCain told supporters in Scottsdale, Arizona. "And I don't really mind it one bit."
Race set to continue
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. A new round of contests in a half-dozen states are slated this weekend and in the coming week.
The Clinton and Obama camps said they expected the count of delegates for the night to be relatively even. "We think the delegates are going to be very close and we may end up with an edge there," Obama campaign manager David Plouffe said.
McCain nearly tripled Romney's delegate count, with 378 to Romney's 127 and Huckabee's 91, MSNBC said. That was well short of the 1,191 needed to win the nomination.
McCain, who lost the Republican primary race in 2000 to George W Bush, has struggled to win over conservatives in the party, who have been unhappy with his views on immigration, tax cuts and campaign finance reform.
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.
About 44 per cent of Republican voters preferred a candidate who shared their values, while one-quarter wanted a candidate with experience.
More than half the total delegates to the Democratic convention in August and about 40 per cent of the delegates to the Republican convention in September will be apportioned in Tuesday's voting.
Economy tops issues
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.
Obama scored victories in Georgia, Delaware, Alabama, Kansas, North Dakota, Connecticut, Utah, Minnesota, Colorado, Alaska, Idaho and his home state of Illinois. Clinton won Arizona, Oklahoma, Tennessee, Arkansas, Massachusetts, California, New Jersey and her home state of New York.
The Democratic rivals were in a tight race in Missouri, where some US news organisations initially projected a win for Clinton, and other outlets later predicted Obama would win.
Because Democrats distribute delegates in proportion to their vote statewide and in individual congressional districts, candidates can come away with large numbers of delegates even in states they lose. Aides for both campaigns predicted the contest would continue for weeks or months to come.
McCain won in California, Connecticut, New Jersey, Delaware, New York, Oklahoma, Arizona, Illinois and Missouri. Huckabee, a Baptist preacher and former Arkansas governor, won in Alabama, Arkansas, Tennessee, Georgia and West Virginia. Romney won North Dakota, Minnesota, Montana, Colorado, Massachusetts, where he served as governor, and Utah, which has a heavy concentration of Mormons. Romney would be the first Mormon president. "It's not all done tonight. We're going to keep on battling," Romney said in Boston.
Huckabee's wins were fueled by strong support from evangelical Christians, and he split votes with Romney among conservatives unhappy with McCain's stances on immigration, tax cuts and campaign finance reform.
"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."
Clinton, a New York senator and former first lady, was battling a wave of momentum for Obama, an Illinois senator who would be the first black U.S. president. Obama has surged in national polls on his message of change.
"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.
Obama continued his strong showing among black voters but also expanded support among whites, winning 40 per cent in Georgia, exit polls said. Clinton, who would be the first female US president, won heavy support from women, exit polls showed.
She and Obama had split the first four significant nominating contests in January and spent heavily on advertising across the country.
(Additional reporting by Jeff Mason, Claudia Parsons, Steve Holland, Ellen Wulfhorst, Andy Sullivan; editing by Frances Kerry)