WASHINGTON (Reuters) - The biggest day of U.S. presidential nominating contests kicked off on Tuesday with Democrats Barack Obama and Hillary Clinton battling for advantage and Republican John McCain aiming to knock Mitt Romney out of the race.

Republican Mike Huckabee struck the first blow on Tuesday with a win 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 candidates for the November presidential election.

Clinton, a New York senator, was hoping to hold off a late surge by Obama, an Illinois senator who has almost caught 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 percent of the Republican delegates are up for grabs. Georgia is the first state to end voting at 7 p.m. EST (2400 GMT).

Huckabee, a Baptist preacher and former Arkansas governor, won in the second round of balloting at the West Virginia Republican convention after Romney led on the first ballot. Huckabee was aided by McCain voters who switched to him to deny Romney a victory, drawing a protest from Romney’s camp.

“This is what Senator McCain’s inside Washington ways look like: he cut a backroom deal with the tax-and-spend candidate he thought could best stop Governor Romney’s campaign of conservative change,” Romney campaign manager Beth Myers said.

McCain criticized Romney for complaining.

“Generally speaking, rather than blame it on someone else, I suggest that he move on,” McCain told reporters. “It’s a bit insulting to Gov. Huckabee, who won that, by suggesting such a thing.”

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.

BIGGEST PRIZE

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.

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Among Democrats, the Reuters/C-SPAN/Zogby poll showed Obama opening a 13-point lead on Clinton in California, where polls close at 11 p.m. EST (0400 GMT on Wednesday). Other opinion polls showed a much tighter race in California, and close Democratic battles 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.”

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Clinton voted in suburban New York, accompanied by her husband, former President Bill Clinton. Obama voted in Chicago and then settled in for a possibly long night.

“I think everybody is flying blind on this one,” he told reporters, saying it was much harder to judge the outcome in so many states.

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.

DUELING COMMERCIALS

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.

He then headed across country to California for one final campaign stop before returning to Arizona for the evening. “We’re going to do well here, we’re going to get the nomination,” he said in San Diego.

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.

(Additional reporting by Jeff Mason, Claudia Parsons, Steve Holland, Ellen Wulfhorst, Andy Sullivan; editing by Eric Walsh)