Perdue, who had been leading in the polls, and Kingston, who dominated in fundraising, secured their spots with the help of a major TV advertising blitz in the final few weeks and left the state's sizeable tea party crowd without a favored candidate. In unofficial returns, Perdue had 31 percent of the vote to Kingston's 26 percent with 90 percent of precincts reporting.
Former Secretary of State Karen Handel, who had tea party support but lagged in fundraising, was headed for third with 22 percent. Reps. Paul Broun of Athens and Phil Gingrey of Marietta were in fourth and fifth, respectively. Earlier, Michelle Nunn easily won the Democratic nomination, defeating her three opponents.
A runoff in Georgia occurs when no candidate receives 50 percent plus one vote.
The primary has been closely watched nationally, with Republicans needing just six seats to claim a majority in the Senate. Nunn is considered a formidable opponent, and Republicans can ill afford to lose the seat left open by retiring Republican Sen. Saxby Chambliss. Although the state has voted reliably Republican in recent years, Democrats see an opening with changing demographics in the state — a growing minority population and residents moving in from out of state.
"I want to be the candidate in November," Perdue told a cheering crowd of supporters. "I've begged you for a year, get me into this general election in November because we will not allow (Senate Majority Leader) Harry Reid to have one more vote in this United States Senate."
Perdue, a cousin of former Gov. Sonny Perdue, saw his standing rise in recent weeks due in part to TV ads depicting his four opponents as crying babies who had their chance to fix the nation's problems. Perdue, who cast himself as an outsider, chipped in at least $2.1 million of his own money to his campaign.
Kingston, a longtime congressman, dominated in fundraising throughout the GOP race and drew support from dozens of state and local officials. Of the three congressmen, Kingston was considered the strongest and received the backing of the U.S. Chamber of Commerce, which spent nearly $1 million in TV and online ads promoting him.
Handel also sought to claim the outsider mantle. She built momentum in the final month with the help of a comment by Perdue about her lack of a college degree and endorsements from the likes of former Alaska Gov. Sarah Palin, but her lack of money hurt her ability to match Perdue and Kingston in critical TV advertising.
Kingston had cast himself as a frugal politician, and he and others pounced on the chance to say Perdue would raise taxes when the former CEO said spending cuts alone couldn't fix the nation's fiscal problems. Perdue dismissed the attack as "deceitful."
Gingrey focused his campaign on the metro Atlanta area and his pledge to repeal the federal health care law in his first term "or go home," while Broun sought to line up tea party support with his plan of evaluating legislation by whether it's right/moral, constitutional, necessary and affordable.
Perdue's comment turned off Sant Perez, 55, an Atlanta sales manager for a television station, who initially considered him along with Kingston and Handel. He ultimately voted for Kingston, saying Perdue's comments made him seem elitist.
"I like Handel, but I think Kingston has a better shot" to beat Nunn because of his experience and more moderate views, Perez said.
On Tuesday evening, Nunn celebrated her victory with a few hundred supporters in a hotel ballroom. Her father, a moderate who represented Georgia in the Senate for years, was among the family members by her side.
"We don't know who the candidates on the other side are going to be but we know that these candidates are all in the race to extremes," Nunn said. "They're more interested in scoring political points than in solving actual problems. Those aren't the Georgia values I know."
The other Republicans in the race were patent lawyer Art Gardner and network engineer Derrick Grayson. Neither had raised much money and trailed in the polls. On the Democratic side, former state lawmaker Steen Miles, psychiatrist Branko Radulovacki and ROTC instructor Todd Anthony Robinson were also on the ballot.
Associated Press writers Bill Barrow, Jonathan Landrum, Kate Brumback and Johnny Clark in Atlanta and Kathleen Foody in Marietta contributed to this report.