The biggest day in US presidential nominating contests kicked off on Tuesday with Democrats Barack Obama and Hillary Clinton struggling for an edge while Republican John McCain hoped to knock Mitt Romney out of the race.
Republican Mike Huckabee claimed the first win on Tuesday with a victory in West Virginia, one of 24 states holding nominating contests on "Super Tuesday" that will yield a huge haul of delegates to this summer's conventions to nominate the two candidates for the November presidential election.
Huckabee, a Baptist preacher and former Arkansas governor, won in the second round of balloting at the West Virginia Republican convention as most McCain voters switched to him to deny Romney a victory.
Economic concerns -- plunging housing values, rising energy and food prices, jittery financial markets and new data showing a big contraction in the service sector -- have eclipsed the Iraq war as voters' top concern, opinion polls show.
Clinton, a New York senator, was hoping to hold off a late surge by Obama, an Illinois senator who has almost caught up to her in national polls and leads in several states taking part in the coast-to-coast voting.
"The fact that we've made so much progress I think indicates that we've got the right message," Obama said on NBC's Today show.
More than half the total Democratic delegates and about 40 per cent of the Republican delegates are up for grabs. Georgia is the first state to end voting at 7 pm EST (2400 GMT).
At the West Virginia convention, Romney led after the first round of voting but was short of the majority needed for a win. Romney, a former Massachusetts governor and venture capitalist, told the convention he was the best candidate to handle a troubled economy and uphold conservative principles.
"I'll make sure we reach across the aisle but we don't walk across the aisle," Romney said in response to McCain's touting of his ability to reach out to political opponents.
A new Reuters/C-SPAN/Zogby poll showed Romney leading McCain by 7 points in California, the biggest Super Tuesday prize. But McCain, a senator from Arizona, held commanding double-digit advantages in many of the largest states.
Huckabee aimed for a strong showing in the South with its concentration of evangelical Christians.
Among Democrats, the Reuters/C-SPAN/Zogby poll showed Obama opening a 13-point lead on Clinton in California, where polls close at 11 pm EST (0400 GMT). Opinion polls show a tight Democratic race in many other states.
Clinton and Obama have split the first four significant contests and spent heavily on advertising from coast to coast.
"None of us really understands what the impact of all these contests on one day will be," Clinton said on ABC's Good Morning America.
Clinton voted in suburban New York, accompanied by her husband, former President Bill Clinton. Obama headed back to Illinois after a round of television and radio interviews.
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.
In contrast, many of the 21 Republican contests are winner-take-all when awarding delegates, meaning a strong day by McCain could give him a commanding lead.
McCain predicted victory at an early-morning rally.
"We're going to win today, we're going to win the nomination and we're going to win the presidency," McCain told a crowd of several hundred in New York's Rockefeller Center.
In dueling commercials, McCain and Romney both invoked former President Ronald Reagan in an attempt to question each others' conservative credentials.
The two also clashed after Romney dismissed one of the Republican Party's senior figures, 1996 presidential nominee Bob Dole, who had rebuked conservative talk-show host Rush Limbaugh for criticizing McCain.