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 historic race that could continue to the Democratic National Convention in August.
"Barring a sweep from either candidate, this race is likely to go through the end of the contests on June 3rd," said CBSNews.com senior political editor Vaughn Ververs. "This is the last big primary day with a significant chunk of delegates available, but it's unlikely to be decisive. If not, it may well be the superdelegates who will determine the party's nominee for the fall."
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, who toured the Indianapolis Speedway Tuesday with racer Sarah Fisher, wouldn't make a prediction about the outcome of the primaries.
"Every race is filed with the unexpected. You never know what's going to happen from day to day," said Clinton. "I never make predictions."
Clinton posed for pictures with the racer's pit crew in its garage and Fisher talked about parts of the powder blue car.
"This may be the technology of the future," Clinton said, holding onto a detached high-tech steering wheel.
Asked by reporters what her message was by being at the Speedway, she said: "That we need to get on the track in America."
"If you want to go forward, you put it in D. If you want to go backward, you put it in R," Clinton said.
Fisher, who said she voted for Clinton that morning, piped up, adding: "And just so you know, we don't have reverse in this car."
In Marion County, Indiana's most populous 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," White said.
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.
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.
In Smithfield, N.C., voter Matthew Casey said he initially favored Clinton, but decided a vote for Obama would help end their bruising primary and allow Democrats to start focusing on Sen. John McCain, the certain Republican nominee.
"We've got to end the war - it's killing the economy," said Casey, 47, a healing arts practitioner.
Jim Ellis, 73, a retired architect from Raleigh, N.C., cast his ballot for Clinton.
"I came here not to vote for somebody, but to vote against somebody. I don't think he's qualified and when the cream comes to the surface ... if he's elected president I think he's going to be a disappointment," Ellis said, adding that he was disappointed in Obama's handling of controversial statements by his former pastor, the Rev. Jeremiah Wright.
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 critical to Tuesday's outcome.
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.
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."
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 1745 to Clinton's 1603 in the latest CBS News delegate count.
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 trailed with 257 to her 271 Tuesday.
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.