A tidal wave of advertisements. A field of 18 candidates all sharing the same ballot. A Democratic rising star who came from out of nowhere to threaten a GOP stronghold. And a string of irascible last-minute tweets from a president tuning into the race.

Those factors will collide Tuesday for the special election to represent a suburban Atlanta district in Congress that has fast become the center of the national political debate. And as voters stream to the polls, the question of the night is bound to be whether Democrat Jon Ossoff can pull off an upset victory.

He’s gunning for a majority of the vote, a prospect that was unimaginable when he entered the race in January as a little-known investigative filmmaker who once worked for U.S. Rep. Hank Johnson. But an unprecedented fundraising haul and a jolt from President Donald Trump’s critics put him within striking distance.

Tom Price’s congressional seat is vacant since he accepted a position in the Trump administration.

"It's still within reach," Ossoff said. "Momentum is on our side. But special elections are unpredictable. And we're prepared for any outcome."

Once jittery Republicans sound increasingly confident they can keep Ossoff under 50 percent, thanks in part to a surge of attack ads from national GOP groups that depict him as an inexperienced stooge of U.S. House Democratic leader Nancy Pelosi. Data suggest that conservative-leaning voters kept pace with Democrats in early voting, and GOP campaigns say internal polls indicate a strong Election Day turnout.

“Tom Price’s win underscores just how truly Republican this district is,” Karen Handel, the leading Republican in many of the polls, said of Price’s victory in November with 62 percent of the vote. “You know how much I love football — the ball is in the air now.”

Add to this unpredictable mix last-minute tweets from Trump, who won Georgia by 5 points but carried the district by a narrower margin. The president referred to Ossoff as a “super Liberal Democrat” who would threaten his agenda on immigration and tax policy.

A Democratic victory in the district, which has been in Republican hands for decades, would be an epic rebuke to Trump’s administration.

And his shadow looms large over the race, with some of the 11 Republicans in the contest running as an independent-minded check on Trump and others promising to be his staunchest ally in Congress. Three other leading Republicans — Bob Gray, Judson Hill and Dan Moody — have aimed fire at each other and Handel as much as Ossoff.

The Trump factor was on vivid display at a campaign stop Monday at a Roswell diner, where attorney Deborah McKinley had but one question for Handel: Did she vote for Trump? When Handel answered yes, McKinley sighed in relief.

“My main concern is finding the Trump loyalists who have the chance to win,” McKinley said after her encounter. “And I think she’s got the best shot.”

Trump is also energizing droves of left-leaning voters who want to hand his administration its first electoral defeat.

"He's a dark shadow. I despise Trump. I feel like he's a con man who has manipulated a lot of people," said Peggy Williams of east Cobb County. "And this is a way to send a message."

Ossoff’s best chance to win the contest might be Tuesday’s vote, when he can exploit the vicious Republican infighting. A June 20 runoff between the two top vote-getters looms if no candidate gets a majority of the votes, and Republicans would be pressured to unify behind a GOP candidate.

But Ossoff and his aides said his unprecedented $8.3 million fundraising haul and national attention to the race will keep him well stocked if the race drags into a second round.

Here’s a look at a few of the key 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.

The Trump factor: A handful of Republican candidates have hinged their campaigns almost entirely on their support for the president. Gray, a former Johns Creek councilman, vows to be a "willing partner" of the president, and 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 most 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, 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.