A huge surge of support for Barack Obama has wiped out Hillary Clinton’s once-formidable lead in four of the biggest “Super Tuesday” states, according to new polls that show the Illinois senator riding an ever-greater wave of momentum into the critical nationwide contest.

As the Democratic rivals campaigned across the country and blitzed most of the 22 states voting tomorrow with the most expensive television advertising war in primary history, Mr Obama appeared to have drawn level in California and was running close behind in New Jersey, Arizona and Missouri.

According to yet another poll — a Washington Post-ABC News survey — Mr Obama is now in a virtual tie with Mrs Clinton among Democrats nationally, after months in which the former First Lady held a double-digit lead. “This is a very contested race for the nomination,” Mrs Clinton told Fox News Sunday. Referring to the fact that more primaries and caucuses were being held on Super Tuesday than on any previous single day in a nomination battle, she said: “This is something nobody’s ever gone through before. We’re kind of making this up as we go along.” Both campaigns are now targeting states that vote after February 5 as they prepare to dig in for a delegate battle not seen in a generation.

On the Republican side, John McCain, the clear front-runner, said that he was “guardedly optimistic” about getting the nomination, after victories over Mitt Romney in New Hampshire, South Carolina and Florida. A new national poll put the Arizona senator ahead of Mr Romney by 48 points to 24, with his aides hopeful of wrapping up the contest next week.

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Mr McCain was, however, facing fierce opposition from many conservatives, and Mr Romney spent the weekend capitalising on their unwillingness to back his rival as his best hope of halting his march to the nomination.

Mr Obama’s recent surge comes after record fundraising and a slew of endorsements that continued yesterday, with Robert F. Kennedy’s widow, Ethel, joining the ranks of Kennedys lining up behind him. Maria Shriver, wife of California’s Republican governor Arnold Schwarzenegger and the niece of John F Kennedy, also endorsed Mr Obama last night.

It could spell considerable trouble for Mrs Clinton if his momentum is borne out in voting booths on Tuesday. It will be particularly troubling for the former First Lady if Mr Obama greatly exceeds expectations in California, the biggest prize tomorrow, where 441 delegates are up for grabs. Adding to the sense of unpredictability, the Californian Secretary of State said that up to a fifth of its votes may remain uncounted on Tuesday because of a huge turnout and postal ballots.

The new polls show Mr Obama obliterating what had been the former First Lady’s enormous lead in California. As recently as mid-January she was 12 points up, and led Mr Obama by 25 points there in October.

All the California surveys suggest that Mr Obama has made considerable ground among whites, with whom Mrs Clinton had enjoyed a significant advantage. He is now evenly splitting the white Anglican vote with the former First Lady. She still enjoys far greater Latino support, although La Opinión, the largest Californian Spanish-language newspaper, endorsed Mr Obama yesterday.

Mr Obama has also closed the gap in New Jersey, Missouri and Arizona, other big Super Tuesday states where Mrs Clinton held big leads of between 15 and 20 points only days ago.

Between them, Mrs Clinton and Mr Obama are spending $19 million (£9.5 million) on advertisements in the Super Tuesday states. While they appeared in Delaware and Missouri respectively, Bill Clinton spoke on his wife’s behalf in black churches in Los Angeles. Oprah Winfrey, the daytime television diva who has backed Mr Obama, appeared in California with Senator Edward Kennedy, brother of the late John F. Kennedy, who endorsed Mr Obama last week.

Mr Obama’s aides hope he can emerge from Super Tuesday either splitting the 1,681 delegates on offer or remaining very competitive.

Mrs Clinton still holds significant advantages. All the polls show her with strong leads among women and traditional, blue-collar Democrats, powerful voting groups in the party’s primary contest. Reflecting a split among the Kennedy clan and the family of Robert Kennedy, his son, Robert F. Kennedy junior, endorsed the former First Lady.

The biggest day in the parties' nomination race

What is Super Tuesday?
This Tuesday, voters in 24 of the 50 states select delegates for the two parties' national conventions, where their presidential candidates will be officially chosen. Some 2,075 delegates from a total of 4,049 will be chosen by the Democrats and 1,081 delegates from a total of 2,380 will be chosen by the Republicans.

What is its significance?
It is the most important indicator of public opinion before the presidential election itself in November. Big states such as California will have their say. A decisive victory can win the nomination for a candidate. President George W Bush and Al Gore both won majorities on Super Tuesday in 2000 and then gained their parties' nominations. In 1992, Bill Clinton mounted a comeback on Super Tuesday and went on to serve eight years in the White House.

What are the rules?
They vary by state and between the parties. Most big states select their delegates in primary elections while many rural states use the caucus system of party meetings. Primaries will be held on Tuesday in 15 states, caucuses in the rest.

What makes Super Tuesday different?
Up to now the candidates have been engaged in 'retail' politics, where they can meet voters in diners or front rooms. They are now engaged in 'wholesale' politics where party organisation, advertising and soundbites come to the fore.

Isn't this a crazy way to choose a president?
On the contrary. It may be expensive and lengthy (Obama and Clinton raised more than $100m last year) but the candidates have to prove they have staying power, grit and the intellectual capacity to debate and campaign endlessly. It allows voters an opportunity to watch future leaders under pressure.

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