"It's a test case for how far a Democratic candidate can go in a Republican district on the strength of opposition to Trump", said Todd Rehm, a Georgia Republican strategist who is not affiliated with any candidate in the race. The nearest Republican was former Georgia secretary of state Karen Handel, who won just 19.8 per cent of the vote.
But the Vermont senator did not appear to know much about Ossoff, the 30-year-old Democrat running to win a vacant House seat in Georgia's sixth congressional district.
Republicans called Ossoff's showing a failure to buy the seat, noting that he spent more than $8 million, and predicted Handel will prevail in a one-on-one runoff June 20.
Despite controlling the White House and both chambers of Congress, Republicans have yet to enact major legislation to fulfill campaign promises.
In addition, Ossoff has also used anti-Trump sentiment, with the slogan "Make Trump Furious", to power his campaign, during which he also focused on the growing worry among Republicans that his victory could have a chilling effect on congressional races in future. Ms Handel did not mention Mr Trump during a 10-minute speech on Tuesday night, according to local media. "Glad to be of help!" he tweeted.
"If you run as a Democrat, you're a Democrat", Sanders said. Party officials say they can beat Ossoff once the primary is over and they unite behind a single candidate.More news: IS claims responsibility for Paris shooting
Republicans faced a sobering reminder of Donald Trump's unpopularity Wednesday after a young Democrat forced a run-off election in a conservative congressional district, highlighting the vulnerability of the president's party as opponents draw inspiration for fierce battles ahead.
Handel treated Trump gingerly throughout the campaign in a district he barely won in November.
The Democrat reportedly amassed an astounding $8.3 million from almost 200,000 donors, with over 95 percent of those contributions coming from out of state.
The hotly contested race carried major implications as a gauge of President Donald Trump's popularity - and Trump himself seemed to grasp the high stakes, playing a direct role in its closing days with robocalls and by attacking Ossoff repeatedly on Twitter.
The commercial, combined with an anti-Ossoff ad Wednesday from a GOP group, are a sign of what's to come.
"This is already a victory for the ages". President Donald Trump lost the popular vote to Hillary Clinton by almost 3 million votes, and won the Electoral College thanks to about 70,000 votes across three states.