Tuesday’s special election in Pennsylvania’s 18th Congressional District is coming down to the wire, with Republican Rick Saccone and Democrat Conor Lamb battling for a seat deep in Trump Country.

President Trump won this Pittsburgh-area district by about 20 points in 2016, but it’s Lamb who has run a high-energy race to challenge Saccone, after Rep. Tim Murphy, R-Penn., resigned last year amid a sex scandal. Republicans have privately grumbled that their candidate has been lackluster, and the Democrat could be poised to pull off the upset. Polls will close at 8 p.m. ET.

If Lamb is victorious, it would be the highest-profile warning yet against the GOP ahead of this November’s midterm elections. Not only will Democrats have finally flipped a GOP House seat in a special election, but the win would come in a solidly red area and underscore just how little help Trump might provide the party this fall. And that could trigger even more GOP legislators to head for the exits, adding to an already record number of House Republican incumbents retiring.

Trump visited the district for a rally on Saturday as a last-ditch rescue mission, where he asked his supporters, “Do me a favor. Get out on Tuesday and vote for Rick Saccone.” Trump also referred to the Democrat as “Lamb the sham,” saying that if elected, Lamb would vote as House Minority Leader Nancy Pelosi wants, even though the candidate said he would not support keeping her as leader of the Democratic caucus.

The president’s visit capped off a massive effort by GOP groups to keep the seat in Republican hands. More than $10 million has been spent to support Saccone, about twice as much as has been spent for Lamb. That massive expenditure is despite the fact that the district won’t even exist in its current form come November due to court-ordered redistricting, underscoring just how desperate Republicans are to avoid an embarrassing loss that would portend a growing blue wave.

It would also show that the GOP’s economic argument to voters and touting their tax cuts may not be enough to save them at the ballot box, either. While early ads focused on a fiscal message, closing arguments from Republican outside groups hit Lamb on immigration and crime instead.

Lamb had outside support in the final days from former Vice President Joe Biden, a Pennsylvania native. The young former prosecutor and Marine veteran has honed in on local issues and eschewed most national help, and even Republicans concede he’s been an unusually tough candidate.

If Saccone wins, President Trump would get credit for juicing Republican turnout in the final days.

Data in the last week of the race backed up assertions from the district that Saccone’s fundraising was weak. Of the more than $10 million spent to support his campaign, less than 10 percent was from the Saccone campaign itself. On the Democratic side, about two-thirds of the money spent to support Lamb came from his campaign.

One byproduct of all that money and the television ads that have blanketed the Pittsburgh media market in recent weeks is a lot of confused voters. Pittsburgh public radio station WESA is reporting that election officials in Allegheny County have fielded calls all day from angry would-be voters wanting to know why their polling place was closed. The answer: These people didn’t live in the 18th congressional district.

In Virginia and in special state-level races around the country over the past year, suburban voters motivated by anti-Trump sentiments have shown up in droves to vote for Democrats. One key question is whether or not that happens again tonight.

Republican operatives are particularly interested in the early results from Upper St. Clair, a high-income, high-education Pittsburgh suburb that traditionally votes Republican. “If Lamb wins there, it’s over,” said one longtime Pittsburgh GOP strategist. Bethel Park is another high-income suburb to track for similar trends.

With a strong push from unions like the AFL-CIO and the United Mine Workers of America, Lamb is also attempting to cut into the wide margins that Republican congressional candidates typically rack up in the district’s rural corners, too. Greene County, in the southern end of the 18th, broke for Trump by 40 points in 2016, but Allegheny County Executive Rich Fitzgerald told NPR that recent polls showed the two candidates neck-and-neck. If counties like Greene and Washington are near-even, then Lamb would have a clear shot to win.

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