Highlighting her working-class message, Clinton visited the Indy 500 racetrack in Indianapolis with Sarah Fisher, a female race car driver who has endorsed her.
Asked earlier Tuesday if she would drop out of the race if she lost tonight, Clinton refused to say.
"Politics is unpredictable. So I'm just going to wait and see what the voters have to say," she said.
After five months of bruising primary battles, Clinton had appeared to find a groove in recent weeks, adopting a populist message, defiantly refusing to withdraw from the race, and pushing the party to seat delegates from Florida and Michigan.
She aggressively targeted blue collar and low-income voters, hammering her message that she will fight for them.
"You notice as she campaigns that she drops the ending of words, and becomes 'we're working people,'" said Peri Arnold, a professor of political science at the University of Notre Dame in South Bend, Ind. "She becomes sort of Rosie-on-the-night-shift and stylistically she becomes very attractive to these voters."
Obama has typically fared better among younger voters, people with higher-education, self-described liberals, and African Americans.
Obama, who went from living on food stamps as a child to becoming the first black president of the Harvard Law Review, has at times appeared uncomfortable in recent weeks trying to appeal to white, rural, working class voters by drinking beer and campaigning at construction sites and on factory floors.
"I think a lot of voters don't know what box to put this guy in," said Arnold, who focuses on presidential politics, arguing Obama's "cosmopolitanism" has confused voters.
"There are questions about Obama in many voters' minds about who he really is in terms of his style and his values and that's problematic because, after all, presidential politics is about connecting with the voters and giving voters a sense that they're like them in some way," he said.
Clinton, herself an Ivy League-educated millionaire and longtime Washington insider, has continued to paint Obama as an out-of-touch elitist and a flawed general election candidate. She has repeatedly argued that only she can withstand Republican attacks in the fall and right the wrongs of seven years of the Bush administration.
Despite her criticisms, a New York Times/CBS poll released Monday found Obama leads Clinton in support nationally, with 50 percent support for Obama to her 38 percent.
Despite the negative tone of the campaign, the ongoing Democratic battle continues to energize voters and bring substantial turnout at the polls.
Obama's win in North Carolina comes despite the efforts of former President Bill Clinton, who campaigned persistently in the state over the last three weeks.
" Bill Clinton has been campaigning here pretty persistently for the last three weeks," said Steve Ford, editorial page editor of the Raleigh News & Observer.
In the two weeks since the Pennsylvania primary, Obama and Clinton have sparred over her proposed suspension of the 18-cents-a-gallon federal tax on gasoline for the summer travel season, with the estimated $8 million in lost revenue to be made up through a new tax on oil companies.
Several economists have derided her plan, and Obama called the proposal a gimmick designed to pander to low-income voters.