Barack Obama and Hillary Rodham Clinton traded victories in an epic coast-to-coast Super Tuesday struggle that failed to establish a dominant front-runner in the race for the Democratic presidential nomination. John McCain seized command of the race for the Republican presidential nomination, sweeping a series of delegate-rich primaries across the United States.Super Tuesday was the biggest primary day in US history and offered an opportunity for either Clinton or Obama to establish a decisive lead. Instead, they divided the 22 states and split the number of delegates available within each state, setting the stage for what could be a long fight for the party's nomination.Clinton, seeking to become the first female president of the United States, won the biggest states, California and her home state of New York. But Obama, hoping to become the first black president, won more contests - at least 13 - including important states such as Georgia, Missouri and Illinois, his home state.Neither Clinton nor Obama proclaimed victory."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.Obama was in Chicago, where he told a noisy election night rally, "Our time has come. Our movement is real. And change is coming to America."Missouri was so close that although Obama won the vote count it was likely to be hours before it became clear whether he or his rival had captured a majority of the state's 72 delegates.The Democratic caucuses in New Mexico remained unsettled. Clinton had a 117-vote lead when the party shut down its vote counting operation until 11 a.m. EST.McCain won the Republican race in California, inflicting a crushing blow on his closest pursuer, Mitt Romney. The Arizona senator was almost halfway to the 1,191 needed for the nomination, far ahead of his rivals. He won some of the most populous states: New York, Missouri, Illinois, in addition to California.It was a remarkable comeback for a campaign that nearly unraveled last year."We've won some of the biggest states in the country," McCain told cheering supporters at a rally in Phoenix. An underdog for months, he proclaimed himself the front-runner at last, and added. "I don't really mind it one bit."Romney won in Massachusetts, where he had served as governor, and Utah, where fellow Mormons supported his candidacy. He also won in North Dakota, Minnesota, Colorado, Alaska and Montana.Mike Huckabee, a Baptist preacher and another former Arkansas governor, had a surprisingly strong performance, using his support among southern Christians to win in Georgia, Alabama, West Virginia, Tennessee and his home state. These were the first wins for Huckabee since he stunned the political establishment by winning the Iowa caucuses, the first major contest of the campaign.Neither Romney nor Huckabee appeared able to overtake McCain, but both vowed to stay in the race. "We're going to win this thing," Romney told supporters in Boston.Super Tuesday marked a shift in the months-long campaign to select US presidential nominees. While early contests helped candidates build momentum, Tuesday's votes were about winning the delegates who will select the nominees. Democrats had 1,681 delegates at stake; Republicans had 1,023 delegates.The allocation of delegates lagged the vote count by hours. That was particularly true for the Democrats, who divided theirs roughly in proportion to the popular vote in each state.McCain led with 613 delegates, to 269 for Romney and 190 for Huckabee. It takes 1,191 to win the nomination at this summer's convention in St. Paul, Minnesota.Clinton had 845 delegates, to 765 for Obama, with 2,025 delegates required to claim the nomination in Denver at this summer's convention. Clinton's advantage is partly due to her lead among so-called superdelegates, members of Congress and other party leaders who are not selected in primaries and caucuses - and who are also free to change their minds.Clinton had long been the front-runner in the Democratic race. Her campaign has emphasized her experience as a senator and first lady. Obama, a first-term senator, has offered a spirited challenge, carrying a message of change and hope after years of political gridlock in Washington.Already, the campaigns were looking ahead to contests Saturday in Louisiana, Nebraska and Washington state and February 12 primaries in Maryland, Virginia and Washington, D.C.Overall, Clinton was winning only a slight edge among women and white voters, both groups that she has won handily in earlier contests, according to preliminary results from interviews with voters in 16 states leaving polling places. Obama was collecting the overwhelming majority of votes cast by blacks, and Clinton was gaining the votes of roughly six in 10 Hispanics.Democrats and Republicans alike said the economy was their most important issue, according to preliminary results from election day interviews with voters leaving polling places.Democrats said the war in Iraq ranked second and health care third. Republican primary voters said immigration was second most important after the economy, followed by the war in Iraq.The survey was conducted in 16 states by Edison Media Research and Mitofsky International for The Associated Press and television networks.McCain entered Tuesday's contests with a large lead in national polls. Tempers heated up between McCain and Romney on Tuesday, with McCain attacking his opponent for having a "terrible record as governor," and Romney retorting that he must be a strong contender if he so angers his rival.McCain, a veteran senator and former Vietnam prisoner of war, has campaigned on his national security experience. His reputation as a maverick has won him support from independent voters and moderate Republicans but alienated conservative Republicans who form the party's core constituency.McCain would be a formidable rival for either Clinton or Obama at a time Democrats hope Americans' dissatisfaction with President George W. Bush, a Republican, will help them win the White House.If he should win the presidency, McCain, 71, would be the United States' oldest first-term president when inaugurated.