Obama was looking to shore up his position as the front-runner, while Clinton was seeking another victory to keep her candidacy competitive in a race that is likely to continue into June and perhaps to the Democratic National Convention in August.
Obama began the day by dropping in on the Four Seasons Family Restaurant in the Greenwood, Ind., a suburb of Indianapolis. He walked around shaking hands, then sat at the counter and had an omelet, chatting with patrons on either side.
"I feel good," Obama said when asked about the day's voting. "I think we've campaigned hard. I think it's going to be close. I'm seeing a lot of enthusiasm."
Clinton was more reticent.
"We're just, you know, looking to see what happens," Clinton told reporters on her campaign plane late Monday. "Obviously we hope to do as well as we can."
"We started out pretty far behind with some tough odds ... I never feel confident. I just try to do the best I can. You know, I don't make predictions because it's very unpredictable. And this has been, I think anyone would agree, a pretty unpredictable campaign season."
In Indiana, Marion County Clerk Beth White said many voters already were in line when polls opened at 6 a.m. Tuesday.
"We really do feel today is going to be a heavy voting day, and our inspectors are ready," said White, the clerk in Indiana's most populous county.
Even before the opening of polls at 6:30 a.m. in North Carolina, there were signs of record turnout. Nearly half a million people had already cast early and absentee ballots as of Monday -- more than half the total number of voters who cast a ballot during the 2004 primary.
"I can't remember a primary that had this much excitement," said Gary Bartlett, director of the North Carolina Board of Elections. "It's truly fun to be part of making history, and I hope that this encourages voters to participate in all primary elections."
Obama, who was flying later to North Carolina to await election results in Raleigh, visited a polling place Tuesday morning at Hinkle Field House on the campus of Butler University in Indianapolis, the site of part of the filming of the basketball movie "Hoosiers." Obama, who chatted with voters, said he had hoped to shoot a few baskets while there, but that the nets were up because of an upcoming commencement.
"I might have to take one shot," Obama said, although he left without doing so.
Like marathoners on their second wind, Obama and Clinton had raced for advantage until the final hours of the campaign for the primaries in the two states.
Clinton, at her scrappiest when her campaign is on the line -- which it has been for weeks -- brought a full-throated roar to a series of events Monday in a day of frantic travel spilling into the wee hours Tuesday.
A wealthy inside-Washington veteran, the former first lady worked hard to make common cause with blue-collar voters crucial to Tuesday's outcome.
"I do see you, I do hear you," she told supporters in Merrillville, Ind., speaking at a local fire station as a dozen firefighters looked down on her from the fire truck behind her.
She pressed her proposal for a federal gas tax holiday that Obama has dismissed as a gimmick, one of the few issues where the two Democrats clearly diverge.
"It's a stunt," the Illinois senator said in Evansville. "It's what Washington does."
Obama's stance was backed up by 230 economists who released a letter Monday opposing the temporary tax break, which would take 18.4 cents off the price of a gallon if consumers got the full savings at the pump. The signers included four Nobel Prize winners and economic advisers to presidents of both parties.
Clinton shrugged off the blistering reviews from policy makers, industry experts and editorial writers.
"I believe we should start standing up for the majority of Americans who are paying the outrageous gas prices," Clinton said. "I'm ready to take on the oil companies."
Obama hurtled from Indiana to North Carolina and back. "I want your vote. I want it badly," he pleaded on a factory floor in Durham, N.C., one of many settings drawing the working-class voters he needs.
Obama capped his day with a rain-soaked, get-out-the-vote rally in Indianapolis featuring Motown legend Stevie Wonder, followed by a visit to a factory for the midnight shift change.
Dual victories by Obama would all but knock Clinton out of the race. Polls, however, have found a small edge for the New York senator in Indiana. Obama remains the favorite in North Carolina, though his lead has shrunk.
Altogether, 187 delegates are at stake in the two states, nearly half the pledged delegates left with eight primaries to go before voting ends in a month.
North Carolina and Indiana cannot mathematically settle the nomination. A candidate needs 2,025 delegates to win, and Obama had 1,745.5 to Clinton's 1,608 Monday.
The key to the nomination is held by superdelegates, party leaders who aren't bound by the outcome of state contests. About 220 are still undecided.
Despite a rash of recent troubles and his loss to Clinton in the big Pennsylvania primary two weeks ago, Obama has continued to nibble away at Clinton's lead in superdelegates. He picked up two from Maryland on Monday, leaving him trailing Clinton 269-255.
Clinton's main hope is to persuade most of the still-neutral superdelegates to disregard his lead in the delegate chase and support her instead. Her campaign also hopes to get a boost by getting delegates from Michigan and Florida seated.
Obama easily outspent Clinton in both states while outside supporters threw big money into the contest, too.
The Service Employees International Union, which is backing Obama, spent about $1.1 million in the state, much of it on ads. The American Leadership Project, which has received most of its money from labor groups backing Clinton, spent more than $1 million on ads in Indiana that questioned Obama's economic policies.