The poll, conducted March 8-11, surveyed 503 registered voters in the district. Its recent reliably red status had diminished the drama about local elections.
On Saturday night, President Donald Trump, who won the district's vote by 20 points in 2016, traveled to western Pennsylvania to campaign for Saccone.
Republicans control both the Senate and the House of Representatives.
The polls depends on a model that anticipates a surge in Democrats at the polls. This is a big change from polling taken in February and January that showed Saccone with a comfortable lead that was more consistent with the Republican nature of the district, but it's worth noting that those polls were taken before outside money began pouring into the race and before the race became the focus of national attention.
"He has a tough race", Trump acknowledged to thousands at a raucous rally in Moon Township.
TRUMP: The world is watching.
A president's endorsement and powers of persuasion can be valuable assets for a congressional candidate. A higher-turnout scenario, more similar to a presidential election, produces a 7-point Lamb lead, 51 percent to 44 percent.
"Lo and behold, they're fighting for their lives here", Doyle told AFP, referring to Republicans. I think they want to make.
And beyond that, Trump in Pennsylvania said several headline-grabbing things that had nothing to do with the race and instead completely upstaged the candidate that Trump was ostensibly there to help - from praising North Korean dictator Kim Jong Un to calling NBC News' Chuck Todd a "son of a bitch" to getting behind a deranged proposal to execute drug dealers.
Republicans have pumped millions of outside dollars into the race, overwhelming the Democrats by seven to one, Yenerall said. Rick Saccone will be a great, great congressman.
His army of volunteers has fanned out knocking on doors, pressing into areas once considered so conservative that a Democrat could have little hope of breaking through.
"It's voter excitement that might put Lamb over the top".
The steelworkers union has been joined by members from the teachers, hotel, and postal workers unions in getting out the vote for Lamb.
Differing from the position of the AFL-CIO, the largest federation of unions in the United States which supports Medicare-for-All, Lamb says that single-payer health care system hasn't proven to be effective and prefers to focus on fixing the Affordable Care Act.
Likely voters - who were largely polled before the President's campaign event - said they are divided on Trump's job performance: 49% approve and 49% disapprove.
Much of the President's speech amounted to a victory tour in which Mr. Trump, standing before enthusiastic supporters, praised tariffs he imposed last week. You've got to know, he's an extraordinary person.
"If it's going to the northern part of the district is probably says Conor Lamb supports guns and it's trying to get the sort of suburban progressive Democrats in the northern part of the district to stay home", Putnam, who lives just outside the district and has canvassed for Lamb, told ABC News.
Democrats could be forgiven for fretting about Tuesday's outcome.
However, when it comes to campaigning, Saccone diverges with Trump's successful strategy of running as an outsider, instead boasting of his decades of experience in Pennsylvania politics.
Democrats nonetheless take heart that those candidates performed better in their districts than Hillary Clinton did against Trump in 2016.