If their moods offer any indication, Barack Obama and Hillary Clinton are vying as much for the role of underdog and front-runner as they are for votes Tuesday in Indiana and North Carolina Democratic primaries.

"We're working as hard as we can and I desperately want every single vote here in Indiana and North Carolina," Obama said.

Obama continues to lead in North Carolina, although most polls show Clinton has closed the gap significantly; the candidates are in a statistical dead heat in Indiana.

The latest Gallup daily tracking poll also shows them in a virtual tie nationally -- with Obama leading by 5 percent, which is within the margin of error.

Drawing distinctions between herself and Obama at nearly every stop Monday, Clinton pushed forth a populist message and unleashed a new TV ad that criticizes Obama for his opposition to a gas tax holiday. The ad calls her the "candidate who is going to fight for working people."

At a campaign rally in Highpoint, N.C., Clinton questioned Obama's commitment to fighting oil companies.

"Senator Obama wants you to pay the gas tax this summer, instead of trying to get the oil companies to pay it out of their record profits," she said. "So I believe that we should start standing up for the vast majority of Americans who are paying those outrageous gas prices."

Clinton also seized the opportunity to attack the presumed Republican nominee, John McCain, who also supports suspending the gas tax.

"Senator McCain wants you to have a gas tax holiday, but he doesn't want you to pay for it," Clinton said. "Well, we've been down that road. We cannot increase the deficit. We cannot take money from the highway trust fund. That would be a mistake."

Meanwhile, Obama stumped through North Carolina before heading to Indiana, where he sought out white, blue-collar workers.

In response to Clinton's new ad, Obama unveiled one of his own -- a 30-second TV spot called "Hometown" that criticizes the New York senator for offering "more of the same old negative politics."

Obama also secured three more superdelegates, bringing his total number to 255, only 14 behind Clinton's 269. Obama leads Clinton overall in the total number of delegates, 1743 to 1607. Indiana and North Carolina have 72 and 115 pledged Democratic delegates respectively.

Each candidate has rejected the idea that the nomination contest will be over following Tuesday's outcome; both candidates planned to spend Monday night in Indianapolis.

Obama attended a rally accompanied by music legend Stevie Wonder in the city's downtown district. Clinton helda late-night rally in Evanston, Ind., before traveling to the state capital.

During a breakfast stop at a Labor Temple in Evansville, Ind., Obama acknowledged the close poll numbers in Indiana.

"This is gonna be a tight election here in Indiana, every poll shows it is a dead heat," he told the crowd. "We need every single vote, so you guys are pretty persuasive, I need you to tell your members that this is something worth fighting for and that they need to come out and vote, and vote for me," he said.

He also addressed his opposition to a gas tax holiday.

"It's a stunt, it's what Washington does," he said at a construction site at the University of Evansville. "This isn't the first time it's been proposed. It's proposed every two years."

Obama, who over the weekend brought his daughters onto the trail for the first time in four months, suggested to the crowd of construction workers that a second stimulus check would be a better solution for short-term relief.

"That would help people immediately relieve some crisis and that's money that you know won't be sopped up by oil companies," he said.

But if energy is key, Clinton is the one who appears revved up for the race to the finish. Obama suggested that the deflating few weeks of campaigning is par for the course, and repeated his wife Michelle's claim from last week that he would be sailing to the end.

"Once you're the front-runner," he said, "then it is, I think, the obligation of the candidates who are behind you to whack you over the head and the press is happy to oblige."

Fox News' Aaron Bruns and Bonney Kapp and The Associated Press contributed to this report