The nationally-watched election to represent a suburban Atlanta district kicks off on Tuesday in Georgia, and what was once seen as a sleepy all-Republican affair has turned into a decidedly closer contest.

Democrat Jon Ossoff has emerged as the unquestioned leader in the 18-candidate field, and he's aiming for a majority vote to avoid a June 20 runoff. But Republicans - there are 11 in the race - are increasingly confident they can hold him below the 50-percent mark.

It is one of the first congressional votes since Trump's victory, and a Republican defeat would widely be seen as a rebuke to his administration.

Here’s a look at a few factors that could decide the vote:

Turnout: As pollsters and pundits tracking the race are quick to say, anyone who says he can predict its turnout is probably bluffing. Nearly 55,000 voters have already cast ballots, and the national attention, the Trump factor and the enthusiasm around Ossoff's campaign could lead to a dramatic uptick in Election Day voting.

That works both ways: Analysts studying early-voting patterns say a tremendous amount of Republican-leaning voters have yet to cast ballots, and GOP campaigns say the overwhelming majority of the voters they’ve contacted indicate they’re waiting until Tuesday to vote.

Pollster Mark Rountree of Landmark Communications said his research found that there are 77,000 voters in the district who cast ballots in the past two GOP primaries. On the Democratic side, that number is just 17,000.

“It tells you that Republicans have a huge potential upswing,” he said. “But so far Democrats are battling hard to get their votes out and are having reasonable success.”

Changing the electorate: Ossoff's staggering fundraising haul has allowed his campaign to target beyond a smaller base of traditional Democratic voters — an essential task if he aims to win long-held GOP turf. Democrats who rarely vote in primaries or special elections are getting personalized flyers; some are receiving multiple mailers a day.

He's also seeking votes from two segments of the electorate who might be more peeved by Trump: college-educated women — a powerful bloc in the affluent district — and millennials. Tepid support from women in the 6th District in November helped drag down Trump's numbers. And polls show Ossoff is winning younger voters by wide margins, although they are also typically the least reliable voters in special elections.

Pay close attention to how the overall Democratic field performs. Even if Ossoff falls short of an outright victory, Democrats will declare a win if he and the other four candidates combined can top 50 percent. But if they lag behind Hillary Clinton's performance in the district - she notched about 46 percent - Republicans will paint him as another in a line of Democrats who can't get it done at the ballot box.

The Trump factor: A handful of Republican candidates have hinged their campaigns almost entirely on their support for the president. Bob Gray, a former Johns Creek councilman, vows to be a "willing partner" of the president, and Dan Moody, a one-time state senator, said he will fight for Trump's agenda.

Trump's late tweets could boost Republican turnout — and aggravate Democrats looking for a late edge. Trump won the district with 48 percent of the vote, and the Republicans running as his loyalists hope to land a runoff spot by locking up much of that bloc. And polls show despite his struggles in the district in November, a majority of GOP voters give the president sound approval ratings.

A county-by-county fight: The district encompasses only a chunk of north DeKalb County, but it's also the bluest part of the territory. Ossoff is hoping to run up the score in this part of the district, so long as he can consolidate votes from the other four Democrats in the race. Another boon for his campaign: Much of that area had limited access to early-voting sites, which could lead to a spike in turnout for him Tuesday.

North Fulton County is home to some of the district's most conservative turf, but it's also the headquarters of three top contenders feuding for the same slice of the electorate. Gray, Karen Handel and Moody all live within a few miles of each other — and all are warring with each other. Turnout could be slightly higher there, too, thanks to city elections for city council members in Johns Creek and Roswell.

East Cobb County had long been the political power center of the district until Price's 2004 victory. Some Cobb Republicans are eager to reclaim the seat and have lined up behind Hill, the only top contender from the county in the race. Although he's struggling in the polls, strong support from Cobb could help him emerge from the pack. And turnout could also get a boost from an eight-candidate race to replace Hill in the state Senate.

More: A closer look at election-eve dynamics in the 6th District race

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