CNN  — 

Republicans are in the final hours of their frantic, all-out bid to save a Pennsylvania congressional district in the heart of Trump country from falling into Democratic hands in Tuesday’s special election.

Democrat Conor Lamb, a 33-year-old Marine veteran and former prosecutor, holds a slight lead among likely voters over Republican state Rep. Rick Saccone, a Monmouth University poll released on the eve of the 18th District election shows.

RELATED: Track results from the special election for Pennsylvania’s open House seat

It’s left the GOP bracing for what would be its most embarrassing – and ominous – loss since Donald Trump took office. Republicans have spent more than $10 million to prevent a defeat in the district, which Trump won in 2016 by 20 percentage points.

If Lamb wins, or even comes close, it will be a signal that Republicans are in danger even in districts where Trump was won handily in 2016, and raise Democratic hopes of capturing the House and Senate this fall. The race would also serve as a model for Democrats running in deep-red districts across the country.

“The world is watching,” Trump said in an appearance in the district Saturday night, upping the pressure for Saccone to deliver on Tuesday.

The race is all about the psychological fallout.

Republicans have suffered major losses during Trump’s presidency, including a blowout in the Virginia gubernatorial race and a loss in Alabama’s Senate race.

But those races were more easily explained. The GOP already knows the suburban areas where they were walloped in Virginia are at risk, and in Alabama, Roy Moore faced credible accusations of child molestation. A series of 39 red-to-blue flips in state legislative races got less national attention, though they continued to show an enthusiastic Democratic base. And the party had managed to hold on in House races, including one in the Atlanta suburbs last spring.

Western Pennsylvania is different.

It’s a white, blue-collar region that has overwhelmingly backed Republicans in recent presidential elections. And, unlike last year’s contests, Republicans now can tout the tax bill that they had hoped would be a persuasive selling point in key races.

Particularly worrying for Republicans is that Lamb has managed to parry the attacks they’ve hoped will work across the map. He aired a television ad noting that he opposes House Democratic leader Nancy Pelosi, who is deeply unpopular with conservative voters.

Lamb has run to the right of most Democrats on a series of issues. He’s said he personally opposes abortion. His first campaign ad featured him shooting a semiautomatic rifle, and he’s opposed new gun control measures in the wake of the Parkland, Florida, shootings. He’s also supported Trump’s steel tariffs, muting what the President hoped would be a major selling point for Saccone.

National Republicans have done everything possible to save the district.

Trump visited twice, including a rally Saturday night where he spoke for 75 minutes and whipped supporters up to back Saccone.

“This guy should win easily,” Trump said, pointing to the size of his own victory there.

Vice President Mike Pence, Donald Trump Jr., top White House aide Kellyanne Conway and other senior Republicans visited the district, too.

House Republicans poured cash onto the Pittsburgh airways and into get-out-the-vote programs. The National Republican Congressional Committee spent $3.5 million. The Congressional Leadership Fund, a super PAC aligned with House Speaker Paul Ryan, spent $3.3 million. The Republican National Committee chipped in $1.1 million and ran a massive field program, while the Trump-aligned super PAC America First Action spent $1 million.

Democrats, meanwhile, spent relatively little: Patriot Majority PAC spent $450,000 and Vote Vets spent $344,000. Several other groups spent smaller amounts, largely on digital advertising. The Democratic Congressional Campaign Committee spent $260,000 on television advertising.

The lower Democratic spending was in part because the party is gearing up for dozens of races in districts with a recent history of voting more Democratic in presidential races this fall – and in part because they didn’t need to spend money to back Lamb. He outraised Saccone by a nearly five-to-one margin, hauling in $3 million by the most recent campaign finance reports filed February 21.

Fearful of a loss, Republicans are downplaying expectations and trashing Saccone as a candidate.

The GOP’s attempt to downplay the race’s importance appears to center on two key factors: Saccone’s weak candidacy, explained through his paltry fundraising, and the district’s Democratic voter registration advantage, even though that edge isn’t reflected in its recent presidential or congressional voting history.

Trump has closely followed the race for weeks and received multiple briefings on it last week, a senior White House official said.

The official said Trump views Saccone as a weak candidate and is aware that he’s struggled to raise money – but that Trump went into his rally Saturday hoping to move the needle for Saccone with his base of blue-collar supporters, particularly in the wake of his steel tariff announcement.

When Trump stood on stage with Saccone in Moon Township on Saturday, he made clear the pressure for a GOP win Tuesday.

“I hate to put this pressure on you, Rick,” Trump said. “They’re all watching because I won this district, like, by 22 points. It’s a lot. That’s why I’m here. Look at all those red hats, Rick.”