Early results showed Obama with a 2-1 lead over rival Hillary Clinton in the first state to close its polls.
CNN projections show a tight three-way race among Republicans John McCain, Mitt Romney and Mike Huckabee.
As polls begin to close, candidates were awaiting results from the 24 states holding primaries or caucuses today.
Republican conservative voters appear to be evenly split between Romney and Huckabee, according to preliminary exit polls of Super Tuesday voters.
Of those who voted for Huckabee or Romney, about 80 percent identified themselves as conservative, according to the polls.
Only 49 percent of McCain’s voters said they were conservative, a sign that the Arizona senator’s efforts over the last week to placate conservative voters has not paid off.
On the Democratic side, early indications suggest it could be a long night, according to CNN senior political analyst Bill Schneider. Those who made up their mind in the last three days appear to be torn between Obama and Clinton.
According to the exit polls, Obama and Clinton are essentially splitting those voters, with 47 percent going for Obama and 46 percent for Clinton.
Huckabee scored the first Super Tuesday victory, winning all 18 delegates at stake in West Virginia — partially with the help of McCain’s backers.
“It gives us some real wind to our backs as we go into tonight,” the former Arkansas governor said after casting his own ballot in Little Rock.
Huckabee won with the support of 52 percent of the state’s GOP convention delegates on the second round of balloting.
Romney, the former governor of Massachusetts, came in second with 47 percent of the vote, and McCain was backed by 1 percent of the delegates.
Romney was ahead in the first round of voting in Charleston but failed to get the majority needed to win.
It appeared supporters of McCain, who placed a distant third on the first ballot, moved over to Huckabee, helping him carry the day.
Romney’s campaign was furious over the “Washington backroom deal.”
“Unfortunately, this is what Sen. McCain’s inside Washington ways look like: He cut a backroom deal with the tax-and-spend candidate he thought could best stop Gov. Romney’s campaign of conservative change,” read a statement from Romney campaign manager Beth Myers.
Huckabee chuckled at the remark.
“I thought he was saying yesterday, ‘No whining.’ So is it no whining or whining? He can’t even keep a straight answer on the ‘whining or no whining’ question,” Huckabee said.
“There was no backroom deal,” he added. “There wasn’t even a front room deal. There was no deal.”
Front-runners McCain and Romney have engaged in bitter exchanges over their conservative records in recent weeks. 
Watch how CNN analysts view the GOP race »
“This is raw politics as it’s really practiced,” CNN senior political analyst Bill Schneider said. “The McCain supporters who were third in the first round decided to throw their weight behind Mike Huckabee in order to stop Mitt Romney from winning this convention. And look at that — they did.”
With 24 states and American Samoa holding primaries or caucuses, Super Tuesday is virtually a national primary day and a pivotal day in the Democratic and Republican races for the White House.
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