Voters in 24 states make their choices in an unpredictable U.S. presidential campaign on Tuesday, with Democrats Barack Obama and Hillary Clinton in a close fight and Republican John McCain aiming for a knockout blow against Mitt Romney.

On the biggest day of voting ever in a U.S. primary race, candidates in both parties compete on Super Tuesday for a huge haul of delegates to this summer's nominating conventions.

Clinton, a New York senator, tried to hold off a late surge by Obama, an Illinois senator who has cut into her once commanding leads in opinion polls nationally and in some states 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, and the question is are we going to be able to pull some states out, Obama said on NBC. No matter what happens though, we're probably going to see a split decision tonight.

More than half of the total Democratic delegates and about 40 percent of the Republican delegates are up for grabs on Tuesday. Georgia is the first state to end voting at 7 p.m. EST, although West Virginia Republicans will make their choices at a convention earlier in the day.

Opinion polls show a tight Democratic race in many states, but a Reuters/C-SPAN/Zogby poll showed Obama opening a 13-point lead on Clinton in California, which alone has 441 delegates to the nominating convention -- more than one-fifth of the total needed to win.

U.S. stock futures fell slightly, reflecting caution before Super Tuesday voting.

Among Republicans, McCain had solid leads in most of the big battleground states. But McCain, an Arizona senator, and Romney, a former Massachusetts governor, changed their plans so they could make late dashes to California, where opinion polls showed a tighter contest.

I'm happy that we're doing as well as we are, but this could be a long night, McCain said on NBC.

A new Reuters/C-SPAN/Zogby poll showed Romney up by 7 points in California, although McCain held commanding double-digit advantages in New York and New Jersey.

Clinton and Obama, who had split the first four significant contests, used Monday to hunt for support in delegate-rich Northeast states. Both campaigns spent heavily on final advertising sprees from coast to coast.

It is unprecedented. We are all kind of guessing about what is going to mean because it has never happened before, Clinton said on ABC.

With the pair running close, aides tried to lower expectations and predicted a lengthy Democratic battle extending to Ohio and Texas in March and Pennsylvania in April.

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.

A LONG CONTEST

The nominating rules of our party are really designed to prolong a contest between two strong candidates, Clinton spokesman Howard Wolfson said. Many of us will be making our reservations for Texas and Ohio and perhaps Pennsylvania and beyond that.

Obama campaign manager David Plouffe cited Clinton's once commanding leads in many of the 22 states holding Democratic contests.

We fully expect Senator Clinton to earn more delegates on February 5th and also to win more states, he said in a memo to reporters. If Obama wins a few and stays within 100 delegates of Clinton on Tuesday, he said, we will have met our threshold for success.

In contrast, many Republican contests are winner-take-all when awarding delegates, meaning a strong day by McCain could give him a commanding lead.

McCain said in Boston he hoped to do well enough to hopefully draw this process to a close, but if not we'll be prepared to continue to go out and campaign.

The campaign battle flared again on Monday as McCain and Romney questioned each other's conservative credentials. McCain unveiled a new ad accusing Romney of running in Massachusetts against former Republican President Ronald Reagan's record.

Mitt Romney was against Ronald Reagan before he was for him, the ad's announcer said.

Romney has tried to take advantage of conservative qualms about McCain's views on taxes, immigration and campaign finance reform. He unveiled his own ad saying McCain agreed with Hillary Clinton on topics like immigration, taxes and conservative judges.

Don't we need a leader who agrees with conservatives? the announcer asked.

Former Arkansas Gov. Mike Huckabee, who won the first contest in Iowa, also remains in the Republican race, and has siphoned conservative votes from Romney in some contests. He is aiming for a strong showing in the South.

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