The seat became vacant last year when Gov. Nikki Haley appointed Tim Scott to the state's empty U.S. Senate seat.
Wearing a gray windbreaker, Sanford walked alone up the street and up a flight of stairs to the building with the polling place in Charleston's historic district.
"Casting your vote wasn't that hard," he laughed, but then added "it's a very significant race for me in a lot of different ways."
It's his first election since disappearing while governor in 2009 and then returning to admit an affair with an Argentine woman. They are now engaged. Sanford said after voting that life can be a series of course corrections and that he hoped voters gave him a second chance.
"We all hope for a second chance. I believe in a God of second chances," Sanford said. "On a professional level, we have had a couple of months to talk about the issues. In that regard it has been a treat and a blessing."
Sanford, who spent months apologizing to groups around the state after he revealed his affair, said when he announced for his old congressional seat that the apology tour was over. Known for his frugality as both a congressman and governor, he has been spending the campaign talking about getting the nation's fiscal house in order. Sanford held the seat for three terms in the 1990s.
With Sanford's campaign war chest and name recognition, Tuesday's race was largely for second place. With so many candidates, an April 2 GOP runoff was virtually assured.
Minutes before Sanford voted, State Rep. Chip Limehouse cast his ballot at the same polling place. Limehouse, who has spent almost $500,000 on the race, said he was sure Sanford would make the runoff and hoped he would be in second place.
"Purely by name ID, the governor has an advantage going into today. I'm not sure that goes past today," he said.
Turner was optimistic after voting at an armory in nearby Mount Pleasant.
"This race has been exciting all along because we started at zero," said Turner, making his first run for political office. "We have made our way as high as you can go in this race because you're not going to pass Sanford in the primary."
For Colbert Busch, who faced perennial Democratic candidate Ben Frasier, the race was the fulfillment of a dream she has had since a young child.
"What an incredible opportunity. God bless America that we can do this," she said, adding that if she won, she would have two weeks to concentrate on the campaign while the Republicans in the runoff battle each other. "That is a real advantage."
Turnout was low, as expected in a special primary.
Chris Whitmire of the South Carolina Election Commission said anything over 20 percent of the 454,000 registered voters in the district would be considered a good turnout.
Copyright 2013 The Associated Press.