ATLANTA – Results in an Atlanta congressional race that dominated the nation's political headlines on Tuesday were delayed for several hours as the Fulton County board of elections "suffered a rare technical error" with a memory card.

The error was discovered by Kennesaw State.

Democrat Jon Ossoff and Republican Karen Handel are headed for a June 20 runoff, in the race that was called by the AP shortly before 1 am.

Ossoff began started the evening with a commanding lead of 71 percent of the vote, as the Democrat's DeKalb stronghold reported first. But as the evening progressed, his lead shrank below the 50-plus-one margin needed to win the race outright, as Karen Handel's lead increased over her GOP rivals.

Ossoff, who lives just outside the district and was unable to vote in the election, had dominated the field of four other Democrats and 11 Republicans, in both fundraising and polling. He carried Cobb, Fulton and DeKalb counties by wide margins over Handel.

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In the June 20 runoff, Ossoff will run in a historically Republican district and now faces a single opponent instead of a crowded, divided field, along with what will surely be the full force of the Republican Party which, as a whole, has remained uncommitted until now.

President Donald Trump, who had recorded a robocall on Monday that urged voters to support the GOP, weighed in on the results:

MORE: Trump tweets Ossoff would be a 'disaster' in Congress

Handel rival Bob Gray said his campaign would support her in a runoff against Ossoff:

The 6th district includes portions of Atlanta, Brookhaven, Chamblee, Dunwoody, Doraville, Tucker, Sandy Springs, Roswell, Alpharetta, Johns Creek Milton and Mountain Park. It has been in GOP hands since 1978, and has been held by such GOP stalwarts as Newt Gingrich and Johnny Isakson.

Turnout in Tuesday's special election was about 40 percent, according to the Georgia Secretary of State.

Photos | Election day in the 6th district

Photos | Jon Ossoff election night watch party

PHOTOS: Karen Handel hoping for a runoff