A Monmouth University poll released Monday showed Lamb leading Republican Rick Saccone in a squeaker - 51 to 45 percent (with a 5.1 percent margin of error) - and that's even if there is a very low turnout, a scenario that generally favors the GOP.
The outcome could offer signs of what's to come in November's midterm elections. The polls close at 8 p.m. Results will begin to be reported after that. President Donald Trump won the district by about 20 points in 2016.
If he does, it will sound new alarm bells for Republicans trying to defend their 24-seat House majority. Other recent Republican candidates have also handily won the district. "I've talked to many of these on the left".
The Pennsylvania contest is largely symbolic since the district (which Trump carried by almost 20% points in 2016) will disappear into the mists like Brigadoon when a new statewide redistricting plan takes effect.
Lamb acknowledges the intense opposition to Trump among liberal voters but says the election shouldn't be about Trump. Why are so many people here and across America paying so much attention to this special election in a district in western Pennsylvania?
Centrist Democrat Conor Lamb is facing ultra-conservative Rick Saccone in a special election for Pennsylvania's 18th district, which has been reliably red for 15 years.
"I like what Lamb is doing, what I hear him talking about ... it seems like he's more truthful than Saccone", Larry Callaway, 70, a retired steelworker from Rostraver said after casting a ballot for the Democrat.
After voting Tuesday in Allegheny County, Saccone downplayed the significance of the unusually close race.
The 60-year-old Air Force veteran turned state lawmaker and college instructor enjoys enthusiastic backing from social conservatives who've anchored his state career, and he's perhaps at his most animated when he touts his opposition to abortion rights.
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Democratic voter Brian Konick supported Trump in 2016.
A Lamb upset would boost Democrats ahead of the November midterm elections.
Voters on Tuesday will decide between Lamb, a former Marine and prosecutor, and Republican state legislator Rick Saccone. The political stakes are so high that the race reportedly played into Trump's decision to announce tariffs on steel and aluminum imports, in hopes of stirring enthusiasm among the voters who strongly backed his presidential campaign in a region long associated with coal and steel.
Tuesday's election, then, carries little threat to Trump in the sense that it could swing the balance of power in the House, opening a path to his impeachment. Trump said in a Twitter posting on Tuesday morning. Jobs and wages up.
"This guy should win easily", Trump told the crowd at the rally.
"I believe we have a pretty good law on the books and it says on paper that there are a lot of people who should never get guns in their hands", he said at a campaign event. Trump Jr. said: "That makes a great ad, but we all know what happens when you go to Washington, you eventually have to toe the party line, and that's why it's important to elect Republicans who will work with the president".
As to why no Democrat challenged US Rep. Tim Murphy in 2014 and 2016, that's easy. He resigned last fall. The chairman of the state party said that the district, held since 2003 by a Republican, is a "Democrat district". Republicans have tried to tie Lamb to Democratic leader Nancy Pelosi, but Lamb has said he does not support Pelosi.
The truth that neither party wants to recognize is that a moderate Republican or a moderate Democrat is the formula for election success.
On Outnumbered, panelists debated what Saccone's possible loss means.
The victor replaces Republican Tim Murphy, who resigned in October amid a sex scandal.