In unofficial returns, the candidates each claimed 50 percent of the vote with 99 percent of precincts reporting. Just 2,512 votes separated the two with an unknown number of provisional ballots as well as overseas and military ballots yet to be counted.
Under Georgia law, the runner-up can request a recount if the margin is less than one percent of the total vote.
A smiling Deal met with supporters late Tuesday at his campaign party at the Gainesville Civic Center.
"We're encouraged by the numbers, and we hope they will hold," Deal said.
Meanwhile, a subdued Handel thanked supporters from her campaign party at an Atlanta hotel.
"We are in this fight," Handel told the crowd. "Let's keep the faith, stay optimistic and party on."
The winner will face Democrat Roy Barnes in November.
Barnes -- who already began running his first ad of the general election even as voters were at the polls selecting his opponent -- said Tuesday night that he will run his own race regardless of the outcome of the Republican contest.
"I'm going to be laying out the comparisons and contrasts in this race based on issues not personalities," Barnes said in a telephone interview.
Handel received the most votes in last month's primary, leading Deal by 11 percentage points in the seven-person race. Tuesday's narrow contest suggested Deal had moved to close that gap, drawing support from rural Georgia.
Handel had been counting on strong support from her home base in voter-rich Fulton County, where she led with 71 percent to Deal's 29 percent. The county's election director, Barry Garner, said late Tuesday that every ballot in the county had been counted except for about 1,500 overseas military ballots.
It was not known how many provisional ballots were outstanding.
Deal, 67, and Handel, 48, entered the runoff with little money, but moved quickly to raise donations and keep supporters fired up. Both candidates wooed voters from south Georgia, which had largely backed other Republicans in the primary.
On the campaign trail, Deal cast Handel as too liberal and said her campaign lacked substance. He also suggested Handel's lack of a college degree sent the wrong message to Georgia students.
Meanwhile, Handel repeatedly assailed Deal's ethics and labeled him "a corrupt relic of Washington." During a televised debate, Handel called on Deal to stop "squealing" about negative attacks and put on "big boy pants."
The attacks worked with at least some voters.
Barry Myers, of Savannah, said news of the ethics allegations against Deal helped cement his support for Handel.
"I'm inclined to doubt how ethical he (Deal) has been since he's been in Congress," the 69-year-old retired medical technician said. "I don't know how much is truthful, but I'm so fed up with politicians getting elected and forgetting what the people sent them to do."
But Handel's sharp tone turned off some voters.
"It was an easy choice. I don't like negative campaigning," said Sylvia Small, an Atlanta public relations counselor who voted for Deal. "A campaign should be able to stand on its own merits."
In the July primary, Handel and Deal were the top vote-getters in the crowded Republican field. Neither earned the 50 percent plus one of the vote needed to claim the nomination and avoid a runoff.
The third- and fourth-place finishers in the race -- former state Senate President Pro Tempore Eric Johnson and state Insurance Commissioner John Oxendine -- did not offer their endorsements.
Deal and Handel did tap national political heavyweights to boost their bids in the campaign's final days. Former Alaska Gov. Sarah Palin headlined a rally for Handel on Monday in Atlanta. Former presidential contender Mike Huckabee appeared with Deal in Gainesville on Sunday.
A poll conducted in the days before the runoff showed Handel with a slight edge over Deal. The poll by Mason-Dixon Polling & Research Inc. showed Handel leading Deal 47 percent to 42 percent with 11 percent of voters undecided.
The poll had a margin of error of plus or minus 4 percentage points.
Republican Gov. Sonny Perdue, who is barred by term limits from seeking office, weighed in over the weekend and said the state's fractured GOP will need some time to heal after the divisive primary and runoff campaign.
"I think the Republican Party wins with either candidate," said Perdue, who has stayed neutral in the race.
Elected in 2002, Perdue was Georgia's first Republican governor since Reconstruction.
Only those who cast a Republican ballot in the state's July 20 primary or who didn't vote at all were eligible to vote in Tuesday's GOP runoff. Those who voted Democratic were barred from casting a Republican ballot in the runoff.