An Ossoff win would not tip the balance of power in Washington, where Republicans control the White House and both chambers of Congress.
A Democrat came close to outright victory in Tuesday's closely watched U.S. congressional primary in Georgia, heading to a run-off in a race which Democrats tout as an early test of resistance to President Donald Trump. Handel took second place with 19.8 percent of the vote.
"Onward to victory", Ossoff, a documentary filmmaker and former congressional aide who campaigned on the idea he would be a check against Trump, said Wednesday on Twitter.
Trump took late interest in the election to fill the old House seat of Health & Human Services Secretary Tom Price in recent days, tweeting repeatedly about it and recording a robocall calling on Republicans to turn out for the race. Mr Spicer said Mr Trump could campaign for her "if needed".
Even as he ran a campaign more focused on jobs and shaking up the Washington status quo, Ossoff became a symbol of the anti-Trump movement. "Karen Handel (R) is pro-life and Jon Ossoff (D) supports abortion for any reason".
Mr Ossoff, who ran on a pledge to "Make Trump Furious", portrayed the NY tycoon as a Washington insider.
You just tweeted moments ago to Donald Trump, "thank you for the call this morning".
Winning the June 20 run-off will be a steeper challenge for Ossoff, however, as Handel will nearly certainly benefit from her party coalescing around a single candidate in a conservative-leaning district. Or will Democrats regard it as a disappointment given the ideal storm of factors that seemed to be lining up in Ossoff's favor for Tuesday's vote?
"I'm supporting Karen Handel because she is a strong conservative who will represent the best interests of those in the 6th Congressional District", Deal said. By contrast, the Republican Party mobilised about US$2 million in counter-advertising, portraying the Democrat as an outsider who is "too liberal".
"I would have been more surprised it it hadn't gone to a runoff, with that many candidates", Simas says.
"We have an fantastic chance here, an extraordinary moment for Georgia", Ossoff told campaign volunteers as they headed out for a final round of door-knocking on Monday afternoon.
The move by Ryan - one of the most powerful Republicans in government - to insert himself into a special election in a GOP district speaks volumes about how anxious Republicans are about this race.
Handel moved quickly to unite her fractured party, drawing immediate endorsements from some of her fellow GOP candidates and national party leaders, including House Speaker Paul Ryan.
Republicans believe a two-candidate scenario will benefit them in a district that has been in Republican hands since 1978, when Atlanta suburbanites elected a young congressman named Newt Gingrich.