House Democrats are hopeful they can pull of their first major special election upset on Tuesday, steal a heavily Republican blue-collar seat and deepen the GOP’s sense of dread over the upcoming midterm elections.

Democrats and Republicans involved in the race agree on a few things. State Rep. Rick Saccone (R) has run a lackluster campaign. Democrat Conor Lamb has proven to be a stellar candidate. Liberals’ fury at President Trump is so strong that even in a blue-collar district he won by 20 points his support for Saccone is a two-edged sword. And the race couldn’t be tighter, a warning sign for the GOP no matter who wins given the deeply populist-conservative nature of Pittsburgh’s suburbs and exurbs.

“Conor Lamb has made it very close, and that’s rather remarkable when you consider the district,” Sen. Bob Casey (D-PA) told TPM. “Conor’s been a great candidate and candidates matter, but this does indicate that we can be competitive in states and districts that maybe a year ago or 18 months ago we’d never considered.”

Republicans privately agree, even as they look to pin the blame mostly on Saccone and downplay what it would mean to lose a deep red, culturally conservative district after a brutal loss in an Alabama Senate race late last year, a beat-down in Virginia’s gubernatorial race, dozens of losses of state legislative seats and a number of close calls in House special elections.

“We’re very concerned about this race,” one national Republican involved in the campaign told TPM. “The enthusiasm’s certainly there for Democrats. We’ve seen that in pretty much every race. That said, this is more an issue of a mediocre candidate, and mediocre is a kind way to describe him, versus a very good candidate.”

Public and private polls from both parties have found a margin-of-error race with the wind behind Lamb, who was down a few points in multiple surveys late last month. As Democrats’ hopes build, Republicans have been increasingly vocal about their frustrations with Saccone, a deeply flawed candidate who has paled in comparison to Lamb both in retail political skill and fundraising ability.

While the telegenic young Lamb has impressed even Republicans with his disciplined campaign, and calculated splits with the national Democratic Party, Republicans have blasted Saccone for weak fundraising, an inability to tell his own story as a veteran, his deep hostility to unions in a union-heavy district, and past mistakes like telling a mother whose kid had died of opioid abuse that addiction was a “family responsibility” and it’s not taxpayers’ responsibility to help, a brutal remark in a district where opioids are a serious problem.

Lamb’s huge fundraising numbers have allowed him to spend more than $3 million on TV ads touting his impressive biography as a former Marine and prosecutor, tout his Second Amendment support (even as he backs universal background checks) and promise to support new Democratic leadership and not House Minority Leader Nancy Pelosi (D-CA), diffusing millions of dollars of GOP attacks tying him to her. Saccone hasn’t reached $1 million in TV, leaving it to outside groups to define him, and while national Republicans realized early on Saccone would be a problem and have spent roughly $10 million to tear down Lamb and boost Saccone they admit not much has worked in the race.

In a sign of growing concern, the National Republican Congressional Committee and the Congressional Leadership Fund, the GOP’s main House super-PAC, have pivoted away from their early attacks tying Lamb to Pelosi and blasting him for opposing the Republican tax overhaul, key elements of their 2018 battle plan. Their closing spots have been culture war attacks on sanctuary cities and accusing him of being soft on crime based on misleading attacks on his record as a prosecutor, though Pelosi remains an element of their attacks.

“With all the millions the’ve spent, they’re not anywhere near where they thought they’d be. They’re just dumping in more money because they don’t know what else to do,” United Steelworkers Political Director Tim Waters, whose union backs Lamb, told TPM.

Waters was in Alabama for now-Sen. Doug Jones’ (D-AL) shocking win late last year, and said he’d rarely seen Democratic enthusiasm that strong for a down-ticket race — until Lamb came along.

National Republican Congressional Committee Chairman Steve Stivers (R-OH) refused to weigh in on whose fault it is that the race is so close.

“I’m focused on winning, I’m not going to talk about blame,” he told reporters last week. “I’m not going to do any Wednesday morning quarterbacking until Wednesday.”

The White House also recognizes the threat of losing a seat Rep. Tim Murphy (R-PA) won with ease for years until an embarrassing sex scandal forced him from office.

Trump stumped alongside Saccone on Saturday, blasting “Lamb the sham” and pushing for support of Saccone because “the world is watching.” But he barely mentioned the candidate he was ostensibly there to back, instead spending most of his speech attacking his own possible 2020 opponents and the media, discussing North Korea and unveiling his 2020 slogan (“Keep America great”) and touting his new steel tariffs, a policy that plays well in the district and both Lamb and Saccone support. The White House has also dispatched Vice President Mike Pence, Kellyanne Conway and Donald Trump Jr. to stump in the district.

While Lamb has intentionally kept the national party at arm’s length, he welcomed Vice President Joe Biden into the district last week, who compared Lamb’s call to serve to his own late son’s, Beau.

All this effort comes as the candidates battle for a district that almost certainly won’t exist after this year. The Pennsylvania Supreme Court has struck down the map, and Lamb is likely to run next fall against Rep. Keith Rothfus (R-PA) in a much less conservative district than the gerrymandered one he’s currently trying to win.

“It’s just amazing that both people are running for a [district] that’s going to last for a few months and then they’ve got to run again and not even against each other,” Rep. Bob Brady (D-PA) told TPM.

And while other national Republicans want to blame Saccone for most of the mess, they concede that the perfect storm has hit — and may blow over other candidates even in seemingly safe seats if they’re not prepared next fall.

“It’s no secret 2018 is going to be a challenging year and quality candidates and quality campaigns matter,” said Congressional Leadership Fund spokeswoman Courtney Alexander.