On the eve of the vote, Senator Kerry was the party's clear favourite to take on George Bush in November, with two wins under his belt and a lead in five of the seven states going to the polls today. But political analysts said he would have to sweep the board to put the contest beyond doubt.

A clean sweep looked possible yesterday but far from a foregone conclusion. Senator Kerry was trailing in the polls in South Carolina to Senator John Edwards, who was born in that state, and who has focused much of his time and money there.

Meanwhile General Wesley Clark had a narrow lead over Senator Kerry in Oklahoma, where the former Nato commander has made a determined appeal to the state's high concentration of war veterans.

Another important state, Arizona, could yet provide an upset, with both General Clark and Mr Edwards spending time and money there and the former frontrunner, Howard Dean, possibly benefiting from early voting, allowed under Arizona rules, with the help of ballots cast before his campaign stalled.

Mr Dean has opted to forgo campaigning in today's seven states, hoping instead to stop the Kerry advance in the more liberal northern states, Michigan and Washington, which vote on Sunday, and Wisconsin, which votes on February 17.

However, the former Vermont governor has continued to take a high profile as the only candidate willing to attack Mr Kerry, whom he accused of trading political favours for donations from business lobbyists in the course of his senate career - charges that Mr Kerry has denied.

Today's vote is expected to deliver a fatal blow to the presidential hopes of the fifth-placed contender, Senator Joe Lieberman, who was trailing badly in polls across the country and who has spent his campaign funds.

Senator Edwards has admitted that a loss in South Carolina would probably end his presidential bid, and the same is almost certainly true of General Clark in Oklahoma. Without a significant number of delegates to the national party convention after the first nine contests, it would be extremely difficult for either candidate to continue to raise funds. The Democratic National Committee chairman, Terry McAuliffe, has asked candidates who are still without a win after tonight, to drop out of the race.

"It is within the realms of possibility that Kerry wins all seven states, and then you don't need any analysis," John Zogby, one of America's leading pollsters, said yesterday. "But if he loses a couple of states and does less well than the polls are now showing, he could be vulnerable further down the road."

"If Edwards wins South Carolina and posts second in Missouri and a third in Oklahoma, picking up delegates in all those states, then he can make the case for a solid regional candidacy," Mr Zogby added. "If Clark wins in Oklahoma and comes second in Arizona, he can move on, but it's hard to see where he goes."

In the biggest state voting tomorrow, Missouri, Mr Kerry held a seemingly impregnable lead (35% over John Edwards according to the latest Zogby poll), which would not only give him the lion's share of the state's 74 delegates, it would also burnish his claim to be a truly national candidate.

Missouri is an important swing and bellwether state that has backed the winner in every presidential election over the past century except once, in 1956.

"Missouri has most of the social groups that are politically relevant in the United States, and it has an economy that represents most of the dimensions of the American economy," said John Petrocik, a political science professor at the University of Missouri.

"It's typically one of the target states of both parties. If you do well in Missouri, you're likely to do well elsewhere."

A national poll conducted by Newsweek found that Senator Kerry would be the first choice of 45% of registered Democrats, against 14% for Mr Dean and 11% for Senator Edwards.

A highly significant yet unknown factor in today's vote will be the allegiances of Hispanic voters, particularly in Arizona and New Mexico where they comprise 25% and 42% respectively.

"These are uncharted waters," F Chris Garcia, a political scientist at the University of New Mexico, told the Los Angeles Times. "None of them has any experience with Hispanic voters."

Hispanics officially leapfrogged African-Americans last year to become the nation's largest ethnic minority, making up 13% of the nation as a whole.

During the last presidential election they were far less likely to turn out to vote than any other group apart from Native Americans, but leaned heavily towards the Democrats when they did.

Both Mr Dean and Mr Kerry have been brushing up their Spanish and addressing issues such as immigration reform and health care, and all candidates have been eagerly seeking endorsements from Latino leaders, which are thought to have greater weight than endorsements among other ethnic groups.

"If the efforts are perceived as ... merely pandering to Hispanic voters by throwing out a few Spanish phrases, people could become insulted," said New Mexico governor Bill Richardson. "But if the message is sincere, the use of a little Spanish comes off as a respect for the culture. It's a bridge."