In hindsight, it's easy to pinpoint where Republicans went wrong with Saccone.
Lamb campaigned as a fiscal conservative who strongly favors gun rights and is personally opposed to abortion (though he would vote for abortion-rights policies).
A Lamb win tonight would signal that the GOP is in danger even in districts considered safe for Republicans, raising Democratic hopes of capturing the House and maybe the Senate in November.
Nevertheless, the district has backed Republicans by double-digit margins in the past three presidential elections. Pennsylvania election results will be refreshed automatically every 30 seconds after polls close at 8 p.m. ET.
Over the weekend, Lamb celebrated an endorsement from the United Mine Workers, a union that sat out the 2016 election rather than endorse Trump or Hillary Clinton. Vice President Mike Pence, White House adviser Kellyanne Conway and Donald Trump Jr. also have made cameos. The Pennsylvania Supreme Court redrew the state's congressional districts after it ruled the old map was unconstitutionally gerrymandered.
In a final-hours message on Fox Business Network, Saccone said that he was ready and willing to help Trump. "I've already said ... that I don't support Nancy Pelosi". As a close observer here in western Pennsylvania, it's clear to me that the passion is on the side of the anti-Trump voters who intend to send a message by electing Conor Lamb.
Not only did the president campaign for Moore, tweet for Moore - but Trump essentially saved Moore's campaign after numerous and detailed sexual assault allegations were leveled against the Republican candidate.
AFL-CIO chief Richard Trumka is weighing in on behalf of Democrat Conor Lamb.
Saccone has been a union foe at the statehouse - a break from the union-friendly Murphy, whose labor backing was a key reason Democrats couldn't topple him. If Democrats lose, they'll point to the district's history and call a close race a win for them anyway (though Democrats have flipped 39 state legislative seats since Trump took office, they haven't been able to do it for a congressional seat).
But Murphy, who espoused strong anti-abortion views, resigned last fall amid revelations of an extramarital affair in which he urged his lover to get an abortion when they thought she was pregnant.
While Lamb is depending on union support, Saccone needs support from Trump voters like Judy Wilson.
Cost writes that Saccone has run a lackluster campaign, with a weak grassroots effort, poor messaging, and lousy fundraising. They had anxious that Rep. Karen Handel, R-Ga., was on track to lose a special 2017 election in suburban Atlanta; her four-point victory went on to boost morale so much that she would hold pep talks for Republican candidates.
Lamb, on the other hand, "is the ideal candidate for that seat", said Jack Posobiec, a fellow MAGA-ite and Pennsylvania native, describing Lamb as a "Pro-Trump Dem veteran".
Lamb, meanwhile, has excited core Democrats and aimed for independents and moderate Republicans.
With polls showing a tight race for months, Saccone has implored the GOP-leaning electorate that their choice is about "making America great again", just as the president says. If anything, Cernovich said, after I'd filled him in on the broad contours of the race, it represented an indictment of the G.O.P.'s failure to connect with the Trump base, while expecting them to remain loyal.
"Let's get back to the good times", Bill Chorgo, a Saccone supporter, said. Jobs and wages up. Greene questioned. "Rick Saccone has walked the walk". Trump had appeared at a rally for Saccone over the weekend.
"I've talked to so many of these on the left, and they have a hatred for our president".
At Friday's event with supporters and volunteers at the party's Allegheny County office, which featured a cardboard cutout of Trump, Saccone, 60, dismissed the anonymous complaints about his campaign.
Supplementing that spending, End Citizens United - a PAC devoted to curbing the influence of outside money on elections - spent about $250,000 on Lamb's behalf in late January; the Service Employees International Union Committee on Political Education spent around $112,000; and two PACs, Progressive Turnout Project and Working America, combined to spend nearly $100,000 on get-out-the-vote efforts. That's a key group in this industrial region.
"Mr. Lamb has done what many Democrats have been unwilling or unable to do: speak directly and plainly to voters about their concerns".
But the Republican nominee finds himself risking an upset that would rattle GOP confidence as it tries to defend its House majority.
The victor replaces Republican Tim Murphy, who resigned in October amid a sex scandal.