Republicans are facing a sobering reminder of their president's poor approval ratings, as 30-year-old centrist Democrat Jon Ossoff, a filmmaker and onetime political assistant, clings to the narrowest of leads.
With Democrats potentially striking the first blow against Trump in 2017, the race has drawn substantial national attention - and vast outside contributions to both candidates.
Ossoff and his Republican rival, the former Georgia secretary of state Karen Handel, spent the weekend and Monday crisscrossing the state as the race went down to the wire.
Trump himself appeared at a fundraiser for Handel back in April, praising her as "incredible" and depicting Ossoff as "someone who is going to raise your taxes to the sky and destroy your health care". "Their ObamaCare is dead", Trump tweeted. "Vote now for Karen H". Ossoff won the April open special election, but with 48.1 percent of the vote, he came short of the 50 percent threshold needed to win outright.
Spending in the race could top $50 million, making it the most expensive House contest in USA history.
Ossoff, 30, initially drew national attention with the slogan "Make Trump Furious", inspiring volunteers from as far away as OR to knock on doors and make phone calls for him.
The two campaigns and political groups have bombarded Atlanta airwaves with election advertising. The Washington Post thinks Trump is trying to tell supporters that the success of his agenda requires a Handel win.
Handel has received support from many top Republicans.
The demoralized Democrats, who were supposed to retain the White House, via Hillary Clinton's candidacy, and possibly win the Senate last November, and then accomplished neither feat, need an Ossoff win to claim momentum going into the 2018 midterms.
The much-anticipated election in the Atlanta suburban district between Ms Handel and Mr Ossoff is seen as a test of President Donald Trump popularity.
Tuesday's runoff is the opposition party's third major bite at the apple since Trump took office.
The Republican candidate in Tuesday's special election in Georgia called a super PAC advertisement featuring images of last week's shooting at a Republican congressional baseball practice "disgusting", despite its message to vote for her.
That attack has ratcheted up tensions in the Georgia race.
Whatever the outcome, the race will not significantly alter the balance of power in Washington, where Republicans control the House by a wide margin.
Some Republicans see political upside in the tragedy.
More than 140,000 voters cast their ballots early - an astounding number for a special election, and one that almost matches presidential contests.
Georgia 6th Congressional District hopeful Jon Ossoff is getting help from a fellow Democrat who also tried to pull an upset in Republican territory.