Here’s what we’ll be watching for Tuesday night (and ):
A Tight Race
Special elections can be especially hard to handicap, but , in April, suggests a close result. Mr. Ossoff won 48 percent, but a host of Republican candidates received a combined 51 percent, forcing a runoff.
Mr. Ossoff has pursued a two-pronged strategy, aiming to peel off a fraction of Republican-leaning voters with a sober, centrist message while mobilizing a broader group of moderates and liberals who are infrequent voters at best and seldom turn out in special elections.
He could win by carrying just 3 or 4 percent of the voters who backed Republican candidates other than Ms. Handel in April. He could also win by turning out enough supporters who did not vote in April.
The final polls showed an extremely close race, with neither candidate holding a clear advantage.
A Surge of New Voters
Nearly 150,000 people have already cast ballots in early voting — nearly three times the early vote in April, when only 193,000 ballots were cast over all. Nearly 40,000 people who have voted early in the runoff did not vote at all in April.
Both campaigns have welcomed the additional voters. But the new voters are far younger, somewhat more diverse, and much less likely ever to have voted in a Democratic or Republican primary than the voters who turned out in April. All of which bodes well for Mr. Ossoff.
Expect a Long Night
Georgia often takes a long time to count its votes, and the April ballot was no exception. The first returns — the early votes of people who cast their ballots at polling places, rather than on paper — will not be conclusive, either.
Those early returns will be more Republican this time, because nearly 50,000 Republican-leaning voters who cast ballots on Election Day in April decided to vote early in the runoff. In addition, many Democratic-leaning in-person voters from April chose to vote by mail in the runoff.
As a result, do not expect meaningful clues to the final result until we learn the votes of people who went to the polls on Tuesday.
Saved by Gunman’s Attack?
For all the early and absentee ballots already cast, the race is competitive enough that Election Day could prove decisive. And, perhaps showing how badly they need a lift, some supporters of Ms. Handel have seized on a liberal, anti-Trump gunman’s at a Republican congressional baseball practice last week as a boon, thinking it could jolt at least some complacent voters into turning out for her.
“I think the shooting is going to win this election for us,” said Brad Carver, a Republican official in Georgia at the county and state level.
A little-known conservative group bought a small amount of television time on Fox News over the weekend for showing emergency crews carrying victims of the attack on stretchers. “The same unhinged leftists cheering last week’s shooting are all backing Jon Ossoff, and if he wins, they win,” an announcer intones.
Ms. Handel’s campaign denounced the ad but did not call for it to be taken off the air.
Each Side Badly Needs a Win
Republicans have held the Sixth District for nearly 40 years. A loss would reverberate in Washington, imperiling the party’s already-stalled agenda and prompting some incumbents to retire rather than seek re-election. Most immediately, it would threaten the Republican , which is expected to come up for a Senate vote in the next two weeks. Democrats, already enjoying a strong recruiting season, would see a bumper crop of candidates for 2018.
But Democrats — who have already fallen short in special elections for House seats and , where their candidates faced stronger opposition — sorely need more than a moral victory. They need to show they can compete and prevail in the kind of wealthy, that represent their most promising path to a House majority next year.
What We Know Already
Tuesday’s result may sharply affect congressional recruitment, retirements and fund-raising. But for Democrats’ chances of recapturing the House in 2018, the lesson is the same whether Mr. Ossoff wins with 51 percent or loses with 49.
Mr. Ossoff’s performance has already confirmed that Republicans in wealthy, conservative-leaning districts will be burdened by Mr. Trump’s unpopularity. Previously safe Republican incumbents in the suburbs of Philadelphia, Miami and Orange County, Calif., could all be vulnerable next year.
The bottom line: A close race in Georgia’s Sixth District is consistent with a strong Democratic performance in next year’s midterm elections — strong enough, perhaps, to retake the House. A few thousand votes either way won’t change that.
Tuesday’s Other Special Election
Though the Georgia battle has consumed the country’s political class, another special election on Tuesday w, who represented South Carolina’s highly conservative Fifth Congressional District before he was named director of the Office of Management and Budget.
National Democrats have done little to compete in the district, which remains strongly supportive of Mr. Trump, and Ralph W. Norman, a Republican former state legislator, is widely expected to defeat Archie Parnell, a Democrat and former Goldman Sachs tax expert. But Mr. Parnell had more money to spend than Mr. Norman in the final weeks, and some South Carolina Democrats expect a surprisingly competitive finish.