When former Vice President Joe Biden took the stage last week to campaign for Democratic congressional candidate Conor Lamb, allies of his GOP opponent grew worried.

Many saw Biden as the supreme surrogate — a secret weapon to launch just before Tuesday’s special election in Pennsylvania’s 18th congressional district. The Scranton native and blue-collar champion notched a 67 percent favorability rating in a CNN poll last December and would appear just days before the less-popular President Trump was set to campaign for Republican candidate Rick Saccone.

Speaking to the Washington Examiner last Wednesday, a Republican close to the White House admitted the Lamb campaign was wise to pull Biden out onto the trail to deliver a closing argument for their candidate. “Trump’s visit to Pittsburgh can’t come soon enough,” the source said, claiming the president is “beloved” by Keystone State voters.

The White House and the National Republican Congressional Committee have worked overtime in the lead-up to Tuesday’s election, where Saccone, 66, and Lamb, 33, are running neck-and-neck for a congressional seat vacated by Rep. Tim Murphy — the disgraced Republican lawmaker who resigned last fall after confessing to having an extramarital affair. A final Gravis poll released days before the election found Saccone leading Lamb by 3 percentage points (45 to 42 percent), putting him within the 3.3 percent margin of error.

“If Saccone ends up winning by high single digits, that would make Republicans feel better. But there’s really not much of a reason to think that’s going to happen,” said Kyle Kondik, a campaigns expert at the University of Virginia’s Center for Politics, who moved the race from “leans Republican” to “toss-up” just five days out from the special election.

Republican and Democratic operatives have been quick to claim that Tuesday’s results will offer the clearest indication yet of what’s likely to transpire in the nationwide midterm elections this November.

“If Saccone wins, it clearly was because he was Trump 2.0,” said Charlie Gerow, a Harrisburg-based GOP strategist, who cast PA-18 as a unique experiment for both parties. “Unlike the previous five special elections for House seats against Democrats who basically adopted the ‘Trump-bad-me-good’ mantra, this time you have a Democratic candidate who is trying to be closer to Donald Trump.”

“[Lamb] has said he will not vote for Nancy Pelosi. He has said he is pro-Second Amendment. He has said that he is personally pro-life,” Gerow noted, claiming a loss for Saccone would likely be blamed on “myriad local factors that have relatively little to do with national politics.”

Publicly, White House officials have lauded Saccone as an ideal addition to the Republican-controlled House. First daughter and presidential adviser Ivanka Trump praised the former U.S. Air Force counterintelligence officer for his “incredible work in workforce development” during a tax event last month, while the president called him “a real friend and a spectacular man” during a January rally in Coraopolis, Pa.

But behind the scenes, Republicans inside and close to the West Wing are terrified of losing another battle, particularly in an area Trump carried by 20 points less than a year and a half ago. Failing to hold onto the deep-red district would be an embarrassment to the president, said one administration official, who predicted the staff of Trump’s 2020 campaign would be closely watching Tuesday’s results.

Less than a week before the election, the White House decided to send in reinforcements. Counselor Kellyanne Conway was sent to campaign for Saccone on Thursday, while the president’s eldest son, Donald Trump Jr., was asked to spend the night before the election stumping for Saccone. Just 24 hours earlier, the House GOP campaign arm dropped another $600,000 into the race to flood the airwaves for Saccone, despite having already spent upwards of $3 million.

“It’s a toss-up, more than it should have been,” a Republican close to the race told the Washington Examiner, adding that “special elections run on momentum, and Saccone just does not have the momentum behind him right now.”

The race in Pennsylvania is the seventh congressional special election to be held since Trump took office, and one that many viewed early on as an uphill battle for Democrats. But disappointment in Saccone’s fundraising performance, and hushed disagreements over whether he was the best candidate Republicans could have run, slowly shifted the race toward the center — and then toward Lamb.

“[Lamb] has been running ahead of Hillary [Clinton’s] level in 2016, and this is a district where you would not think Democrats would do that well,” said Kondik. “In the end, I think that’s probably a good result despite whether they win or lose.”

The victor on Tuesday will remain in office until November, when Murphy’s term would normally have ended. They will then have to run again in a newly configured district due to the state supreme court’s recent decision to reverse a Republican gerrymander and redraw the congressional map.

“I am a little skeptical of drawing too many conclusions from this situation because of what the Supreme Court of Pennsylvania has done. This is only for a nine-month term, so it’s going to be a low turnout election,” Gerow said.