24 States to Choose Party Candidates

WASHINGTON -- The biggest day ever in U.S. presidential nominating contests was underway 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.

Twenty-four of the 50 states hold nominating contests for one or both parties on "Super Tuesday," yielding a huge haul of delegates to this summer's nominating conventions to choose the candidates for the November presidential election.

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, tried to hold off a late surge by Obama, an Illinois senator who has cut into her once commanding lead 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," 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.

Romney, a former venture capitalist, told the West Virginia convention that 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," the former Massachusetts governor said.

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 New York and New Jersey and many other of the largest states.

Among Democrats, the Reuters/C-SPAN/Zogby poll showed Obama opening a 13-point lead on Clinton in California. 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 for any of us," 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 that 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 following his appearance on NBC's morning show.

In dueling commercials, McCain and Romney both invoked former President Ronald Reagan in an attempt to question each others' conservative credentials.