Hillary Clinton and Barack Obama head into today's nationwide Super Tuesday contest, when voters in nearly half of America's 50 states go to the polls, both knowing that their historic struggle for the Democratic presidential nomination will keep America and the world transfixed for days and possibly weeks to come.

In an electrifying battle that will see either a black man or white woman as a US presidential candidate, the pair head into their 22-state contest with Mr Obama having pulled even with Mrs Clinton after a fortnight of extraordinary momentum, wiping out her double-digit leads in national polls, and in critical states such as California, New Jersey and Missouri.

A month after their early contests amid the snow and ice of Iowa and New Hampshire - with Mr Obama winning the former and Mrs Clinton launching a stunning comeback in the Granite State - voters speak today coast-to-coast, from frozen mountains in the north to the red earth of Deep South. There are elections in Alaska and Alabama, New Mexico and New Jersey, Idaho and Oklahoma.

Yet, because Democratic Party rules stipulate that the 1,681 delegates up for grabs today are awarded proportionately - a candidate can lose a state but return a similar number of delegates - both campaigns expect the two rivals to roughly split the delegates on offer.

A race like a crackpot Dynasty

The contest between Barack Obama and Hillary Clinton matches any fiction

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It makes for the closest Democratic race in a generation and one that may not be decided until Ohio and Texas vote on March 4 - or even when Pennsylvanians got to the polls on April 22 - a prospect of deep dread for already exhausted aides and strategists. To win, one candidate must reach the magic number of 2,025 delegates.

On the Republican side, John McCain, now the prohibitive frontrunner after his consecutive wins in South Carolina and Florida over Mitt Romney, is hoping to all but clinch the nomination today, a result which would mark one of the greatest political comebacks of modern times. Largely written off just six months ago, he has spent the past 24 hours with money and endorsements suddenly rolling in, travelling on a campaign jet with the food and drink flowing, mobbed by the national press, and appearing at rallies cracking jokes (all of which he has told a a thousand times before). There are 1,023 Republican delegates up for grabs today in 21 contests.

The sheer scale and historic sweep of the nationwide elections - never have so many states held primaries and caucuses on a single day - has taken on all the trappings and razzamatazz of a general election campaign, with celebrity endorsements, multi-million dollar advertising blitzes, and huge rallies that could be mistaken for the final days of a White House battle.

Never before has America witnessed a nominating season like this, particularly the contest between Mrs Clinton - the former First Lady whose husband Bill has at times dominated her campaign - and Mr Obama, a man with a black Kenyan father and white Kansas mother whose was raised in Hawaii.

Nowhere in the world could one see a contest to become a presidential candidate involve Democratic and Republican candidates working collectively to woo Hispanics, women, African Americans, Mormons, evangelicals, fiscal hawks, anti-war liberals, Hollywood stars, blue-collar union members, Wall Street moguls, $10 internet supporters, young voters, pensioners, single-parents and families mired in debt. Never before has a a primary season generated such intense interest. Turnout in most of the states that have voted so far has been unprecedented. Only in the US could a nomination battle elicit such colossal fundraising totals, and the country's first $1 billion election. Never have candidates from both parties embraced the message of "change" so passionately. America has never seen a former president campaign for his wife, turning himself in the process from world statesman back to partisan attack dog.

This campaign, the first time since 1928 that neither an incumbent president or vice-president will contest the general election, has also seen efforts by candidates to become either America's first black president, female president, Mormon president, or in Rudy Giuliani, a thrice-married New York mayor turned president. He is now backing Mr McCain.

Four months ago, Mrs Clinton enjoyed huge poll leads, both in national surveys and state-to-state. Just three weeks ago, she held large leads in California, New Jersey, and Missouri, and a double-digit advantage in national polls. Today, after his big win in South Carolina on January 26, a backlash against alleged racial hardball tactics deployed by Mr Clinton in the Palmetto State, and the endorsement last week of a large swathe of the Kennedy clan, Mr Obama is truly competitive. In January alone, he raised $32 million, smashing all fund-raising records and allowing him to spend at least $11 million in the past week on campaign advertisements in more than 20 states.

In the past two days, while he and Mrs Clinton campaigned from the west to the north-east, Oprah Winfrey, Caroline Kennedy - the late John F Kennedy's daughter - and Senator Edward Kennedy, his brother, all of whom have endorsed Mr Obama, held huge rallies in California. In a surprise appearance in Los Angeles, Maria Shriver, wife of the state's Republican governor Arnold Schwarzenegger, endorsed Mr Obama (her husband has backed Mr McCain). Before a massive and cheering crowd, she said she decided just in the past few hours to come out and back him, because "I thought, if Barack Obama were a state, he'd be California. I mean think about it. Diverse. Open. Smart. Independent. Bucks tradition. Innovative. Inspiring. Dreamer. Leader."

Ms Winfrey, the daytime television diva, has never backed a political candidate before, but told the crowd she had been offended by people who suggested she was supporting Mr Obama solely because of his race. "Don't play me small. I'm not that small. I would never vote for anybody because of gender or race. I'm not voting for Barack Obama because he's black. I'm voting for Barack Obama because he's brilliant."

In New Jersey yesterday, Robert de Niro appeared at a rally with the Illinois senator to back him. Hours later, the Clinton campaign announced the endorsement of Jack Nicholson. Mrs Clinton also became tearful at an event in Connecticut, after an emotional introduction by her former mentor at Yale. Her tearful moment in a coffee shop on the eve of the New Hampshire primary, which seemed to humanise her and galvanise women to her side, is considered a significant factor in her surprise win there. Never before have the moist eyes of a former First Lady been scrutinised and analysed with such intensity.

Mr McCain still faces a concerted challenge from Mr Romney, seeking to become America's first Mormon president. Conservatives have always distrusted the Arizona senator because of his stances on issues such as immigration, taxes and political funding. They are rallying, led by the talk show host Rush Limbaugh, to block his coronation. Yet they have a tough battle. Republican delegates are mostly awarded on a winner-takes-all basis in each state, giving Mr McCain, way ahead in national polls, a big advantage.

Meanwhile, Mike Huckabee, the winner of Iowa, former Arkansas governor and onetime Baptist minister, is staying in the race, robbing Mr Romney of crucial conservative votes in the South. He is till stumping with his star endorser: Chuck Norris, the former world karate champion turned movie action hero.

Nowhere in the world could a former karate champion suggest like Mr Norris did last week that a candidate - Mr McCain, aged 71 - might die in office, and Mr McCain retort that he will send Sylvester Stallone, who has backed him, to beat some sense into the rival action hero.

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