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.