(CNN) Democrats have been desperate to deal President Donald Trump a real setback at the ballot box for months -- and all their hopes are riding on Tuesday's House election in Georgia.

In what's become the most expensive House race in history, both Democrats and Republicans have tons at stake.

A win by Democrat Jon Ossoff will give his party a crucial proof point that Trump's unpopularity is damaging Republicans up and down the ballot -- and make it much harder GOP lawmakers on Capitol Hill to tie their own political fortunes to Trump on health care, tax reform and more. It would also give progressives who pumped $23 million into Ossoff's campaign something to celebrate.

A win by Republican Karen Handel, meanwhile, would give Trump and Republicans confidence in their agenda. And it would deliver a sharp blow to Democrats who had seen the race as their last, best shot at a special election win that would pump their base up and help them draw top-notch candidates for the 2018 midterms.

Ossoff and Handel were the top two finishers in an April 19 primary, and advanced to the June 20 one-on-one runoff. The polls in Cobb, Fulton and DeKalb counties -- the three where the sixth district House seat is located -- close at 7 p.m. ET on Tuesday.

Here are five things to watch in Georgia's special election:

Will Republicans show up? Would they vote for Ossoff?

In ordinary years, this is no swing district. Former Rep. Tom Price -- whose departure to become Trump's health and human services secretary opened the seat up -- won by at least 23 percentage points every time he was on the ballot since 2004. Mitt Romney carried the district by 23 points in 2012.

The only reason Democrats have even an inkling it could be competitive is that Trump collapsed here, besting Hillary Clinton by just 1.5 points last fall.

That all means there are many more people who typically vote Republican in the district than Democrats.

It's created a rare scenario where the huge early vote turnout -- 140,000 people have already cast their ballots, including 36,000 who didn't vote at all in the April primary -- could actually benefit Republicans.

The question is whether these scores of what have historically been reliably GOP voters are separating this race from their distaste for Trump and sticking with Handel -- or have been turned off more broadly by the Republican brand under Trump and are willing to back Ossoff.

The 36,000 voters who did not participate in the primary are perhaps the most baffling to both parties. Both sides have placed major emphasis on turning out voters who participated in Georgia's presidential primaries last year but did not vote in April -- and there are more Republicans than Democrats in that pool of potential voters.

What it all means: No one is quite sure what to expect, aside from a close race.

Democrats' focus: African-American turnout

Democrats have placed a particular emphasis on turning out Atlanta-area African-American voters. The Democratic Congressional Campaign Committee has pumped hundreds of thousands into ads on black radio stations and digital ads, as well as $325,000 for get-out-the-vote mail pieces targeting those voters.

The makeup of the electorate is critical to watch. Ossoff has endorsements from two leading African-American Georgia Democrats -- Rep. John Lewis, a civil rights icon, and Rep. Hank Johnson. But he is also just 30 years old and does not have long-lasting political connections through the district.

To win, Ossoff will need something approaching presidential-level turnout from Democratic base voters -- and African-Americans are a crucial component of that base.

If Ossoff wins: Democrats will eye a majority

Ossoff's campaign has been a testing ground for Democrats' hopes that Trump's unpopularity will allow them to compete for GOP-held seats in suburban areas across America.

Many of those districts are actually less Republican than Georgia's sixth district. What they tend to have in common: Relatively highly educated, wealthy and diverse populations, plus people who -- like thousands in Ossoff's district -- voted for Mitt Romney in 2012 and then backed Hillary Clinton in 2016.

An Ossoff win would be a proof point suggesting that Democrats are on the right track.

It would also be important in two other significant ways: Online, small-dollar fundraising has shattered records so far this year, and an Ossoff win would likely keep that money flowing in. And Democrats are deep into recruiting their crop of challengers for GOP seats in 2018; an Ossoff victory could embolden more top prospects to jump in.

If Handel wins: House GOP leaders can breathe easy

The outcome of Georgia's contest is likely to become a prism through which congressional Republicans view Trump -- a reality with major policy and political implications.

A spate of retirements from nervous incumbents who lack the stomach for a bitter re-election battle could be avoidable: A Handel victory could show worried party members -- particularly those in suburban districts that Democrats are targeting -- that they can still rely on a strategy of turning out their base in Republican-leaning districts, even if Trump is unpopular there. She'll have even helped write the playbook, after relentlessly working to tie Ossoff to Nancy Pelosi and hitting him on national security.

It would also make life easier for congressional GOP leadership, which can't afford to shed many votes from nervous members if it is to advance tax reform and health care legislation in the coming months.

But make no mistake: Republicans are closely watching the results to see just how much of a drag Trump is on Handel. Even a razor's-edge win in a district where GOP congressional candidates typically top 60% would be a stark reminder of the wave potential of the 2018 midterms. After all, 71 incumbent Republicans sit in districts that are -- per the Cook Political Report's partisan voter index -- less GOP-leaning than Georgia's 6th District.

A clear Handel win could show Republican lawmakers that there's no need to distance themselves from Trump -- but anything short of that could send them scurrying from the President.

Was Ossoff too liberal or too moderate?

Ossoff became a fundraising phenomenon because he represented progressives' best chance of swiping a House seat from Republicans early in the Trump presidency. He raised more than $23 million that way.

But if you watch Ossoff on the campaign trail or in TV ads, you'd never know it.

Through the campaign, Ossoff was hesitant to even say Trump's name. Instead, he sold himself as a moderate who would happily work across the aisle, who fretted about deficit spending and who wasn't even sure he'd vote for Pelosi for House speaker.

This reality has Sen. Bernie Sanders' liberal wing of the party concerned that the Democratic establishment is recruiting and running too many moderates -- and the establishment worried that Sanders' insurgency could view a loss as proof the party needs to embrace a much more aggressive, populist, Sanders-like message.

Sanders himself fed this narrative when he pointedly answered, "I don't know," when the Wall Street Journal asked him this spring whether Ossoff is a progressive. "Some Democrats are progressive, and some Democrats are not," Sanders said -- though he later clarified that he does support Ossoff.

This could worsen tensions that already exist between the Sanders-aligned left, which fumed over the Democratic Congressional Campaign Committee's refusal to spend heavily on races in Kansas and Montana, and the DCCC.