12:05 A.M. - And the winner is...
"It took a little longer than we thought, but we did it," Conor Lamb declared at the top of his remarks Wednesday morning after a dramatic night and votes that are still too close to officially call by the Associated Press.
Just a minute earlier, he was announced to the stage as "Congressman-elect Conor Lamb."
Meanwhile, a spokesman for Rick Saccone tells ABC News' Adam Kelsey: "We have no plans to concede — tonight, at least. We want to make sure all the ballots are counted."
12:05 A.M. - Absentee ballots still out
The last two remaining precincts in Westmoreland County have now reported, as well as their absentee ballots, and Conor Lamb’s lead has dropped to 579 votes. Now all 100 percent of precincts are reporting, but we are still waiting for absentee ballots in Greene and Washington counties and provisional ballots in all four counties. Washington County ballots are expected to be counted Tuesday night and Greene County absentee ballots are expected to be counted Wednesday.
Provisional ballots could take up a week to be counted. There are an unknown amount of provisional ballots to be counted.
12:02 A.M. - Republicans indicate they will contest a close race
The National Republican Congressional Committee said the race was too close to call and indicated they would contest a close race.
"This race is too close to call and we’re ready to ensure that every legal vote is counted. Once they are, we’re confident Rick Saccone will be the newest Republican member of Congress," said NRCC Communications Director Matt Gorman in a statement.
"The NRCC is proud of our efforts in this race to promote Rick’s message, motivate Republican voters, and hold Conor Lamb accountable."
11:46 P.M. - Democrats declare victory, even though the race hasn't been called
The Democratic Congressional Campaign Committee has declared victory, even though the race has not been officially called.
“I want to congratulate Conor Lamb and his team of grassroots supporters on an incredible victory. I’m excited to have Conor as a colleague and look forward to working with him on the critical issues facing our country,” DCCC Chairman Ben Ray Luján said in a statement.
“These results should terrify Republicans. Despite their home field advantage and the millions of dollars outside groups poured into this race, Republicans found that their attacks against Conor, including their unpopular tax scam, were not believable,” he said.
The key line of the night from Lujan, which shows the Democrats’ thinking on how tonight will affect the battle to control the House: “There are more than one hundred districts more favorable for Democrats than this one and we look forward to competing hard in every single one.”
11:40 P.M. - "Hang in there."
Rich Fitzgerald, the county executive for Allegheny County, spoke at Conor Lamb’s headquarters Tuesday night and told the crowd of supporters to hang in there.
“We have a still few votes to count,” he said. “Conor is just so grateful for everybody’s support. This campaign is really about the grassroots.”
He also pleaded with the crowd to stay at the party.
“Hang in there we still have a little bit of a ways to go. We don’t want anyone to go home yet.”
Meanwhile, Rick Saccone addressed his supporters and told them they would emerge victorious.
“We’re going to fight all the way to the end. You know I never give up,” he said.
He thanked them for coming out to the polls and urged them to go home if they wanted as they did not know how late the night would go.
“We’re going to keep fighting. Don’t give up,” he said. “We’re going to win it.”
11:39 P.M. - 99 percent of precincts reporting
With 99 percent of precincts in the district reporting, the two candidates are running neck and neck. Fewer than 850 votes separate the two of them.
ABC News estimates the two outstanding precincts we will only yield about 1,000 votes. Another few thousand absentee and provisional ballots are still being counted.
11:22 P.M. - The Associated Press will not make a call tonight
The Associated Press is not declaring a winner in Tuesday's special election for Pennsylvania's 18th Congressional District because the race between Democrat Conor Lamb and Republican Rick Saccone is too close to call.
Under Pennsylvania state law, supporters of the trailing candidate can request a recount.
11:19 P.M. - The excitement will continue
Washington County has decided they will count their absentee ballots tonight: 1,195 absentee ballots there need to be counted. The clerk says it will take between one and three hours to count them. Stay tuned!
10:57 P.M. - Less than 800 votes separate candidates
While the race remains a dead heat at 50-50 with less than 800 votes separating the two candidates, we are still waiting on 7 percent of precincts from Washington County and one percent of precincts in Westmoreland County plus the absentee and provisional ballots.
10:56 P.M. - Saccone and his family praying
A Saccone family friend told ABC News' Becky Perlow that Rick Saccone is upstairs, in the area above his campaign headquarters, surrounded by friends and family. They’re all sitting around, telling stories and sharing pictures on their phones of each other’s kids. Earlier they stood around holding hands and praying.
10:25 P.M. - Democratic turnout is way up
Turnout in the most Democratic county, Allegheny, was 6 percentage points higher than the other three more Republican counties that make up the 18th Congressional District. But, a bigger part of the math that made this race so tight was that Conor Lamb also seems to have swung votes and outperformed Hillary Clinton across the board, doing 10 percentage points better than the 2016 Democratic presidential nominee.
10:07 P.M. - Saccone and Lamb are neck and neck
With more than 95 percent of precincts reporting in PA-18, Rick Saccone and Conor Lamb are separated by just over 1,000 votes in the raw total.
About 6,000 absentee ballots have been returned in PA-18 and none have been counted so far. Allegheny and Westmoreland counties expect to count them later tonight in their final report, but Greene and Washington counties say they will not count them until tomorrow.
9:45 P.M. - Saccone inches forward in vote count
Our ABC News analysis shows this will, in fact, end up being an extremely tight race. Democratic Conor Lamb right now is tracking at the number he needs in the raw vote tabulation in each county in order to tie. The ultimate outcome is still in doubt, but what’s clear is Lamb has strongly and consistently outperformed Hillary Clinton in a traditionally red district Donald Trump won by 20 percentage points.
With more than 70 percent of precincts reporting in the district, Republican candidate Rick Saccone has made up ground on Lamb.
We estimate there are over 40,000 votes out in Westmoreland and Washington counties and about 18,000 votes out in Allegheny county and as a result, we expect the current five-point margin appearing in vote totals to shrink more and tighten.
Westmoreland is the second largest county in the district, which Trump won in 2016 by 34 percentage points. Saccone is currently leading Lamb by 14 percentage points, contributing to the narrowing of results.
Lamb is obviously doing better in the most Democratic of precincts, but where Lamb is outperforming Clinton the most right now is in actually the most Republican precincts.
9:31 P.M. - Saccone watching results, not TV
Several of the voters at Saccone headquarters are wearing Trump hats or pins. And a spokesperson for Saccone tells ABC News' Becky Perlow that the candidate is upstairs, above the rally with his wife, son Matt, Matt’s wife and his two grandkids. They’re not watching the news. They just keep refreshing the Pennsylvania Secretary of State website, watching the results.
9:13 P.M. - Saccone making up some ground
With 35 percent of precincts reporting, Republican candidate Rick Saccone is making up ground.
Precincts are starting to report in Westmoreland, the second largest county in the district, which Trump won in 2016 by 34 percentage points. Saccone is currently leading Lamb by 10 percentage points, narrowing the gap we’ve seen so far in the raw vote.
9:04 P.M. - Lamb still outperforming Clinton
Lamb is also leading in the vote tabulation in Washington County and strongly outperforming Hillary Clinton in the county. In the 2016 race, Trump bested Clinton by over 25 percentage points, currently, Lamb is leading Saccone, although these are still the very first votes to come in from the county.
9:01 P.M. - Saccone voter praises candidate's support for Trump
Holden Sczerba, a 19-year-old Duquesne University College Republican vice president, told ABC News he supported Saccone because he's the best candidate to support Trump's agenda. He also mentioned the tax cuts Republicans passed, which is an issue the GOP hopes will get them traction with voters in other contests.
ABC News' Becky Perlow reports from Saccone's headquarters that the energy in the room is nervous excitement. They all expect Saccone to win, given the district's red history. Most of the voters here are personal friends of Saconne's — people who have known him for 15, 20+ years, and have shared stories about him as a friend, a military veteran, and more.
Watch more voter interviews on our live stream.
8:57 P.M. - Lamb is outpacing Clinton's 2016 performance
Lamb is currently leading in the raw vote tabulation, but most of these numbers are from Allegheny county, the largest and the most Democratic of the counties in the 18th Congressional District. Lamb is outperforming Hillary Clinton in both Allegheny, the largest county in the district, and in Greene, the smallest, despite Saccone currently leading in Greene. In 2016, President Trump won Greene count by 43 points.
8:48 P.M. - First set of numbers rolling in
The first numbers are coming in and Democrat Conor Lamb is out-performing Hillary Clinton in the first precincts to report. We should caution these are the very first results.
The first votes posted more than 45 minutes after the polls closed and the numbers are changing by the minute.
8:46 P.M. - And what is Trump doing tonight?
For those curious as the Pennsylvania 18th Congressional District results continue rolling in, President Donald Trump is currently attending a fundraiser at the Beverly Hills estate of Ed Glazer, who is the owner of the Tampa Bay Buccaneers.
Tickets for the fundraiser range from $35k-250k per person for the Trump Victory committee, the joint fundraiser between the Trump reelection campaign and the Republican National Committee.
The committee expects the event will raise $5 million.
8:15 P.M. - Conor Lamb supporters hopeful
At Conor Lamb headquarters in Canonsburg, Penn., a roar went up at 8 p.m. when polls closed, a major milestone for so many of the party's attendees who have braved the western Pennsylvania cold all winter to canvass for the political novice. All election night parties begin as spirited affairs but there's a notable buzz in the air here and campaign staffers have been doing a poor job of hiding their grins.
ABC News is also at Republican Rick Saccone's headquarters. Check out our our live stream here for more.
8:00 P.M. - Polls have closed in Pennsylvania
Polls have closed in Pennsylvania.
ABC News is live from the candidates' headquarters. Watch our live stream here.
And keep an eye on ABCNews.com for results as the votes come in.
6:25 P.M. - The sights and sounds of election day
Conor Lamb took his grandmother to the polls to vote this morning in Carnegie, Pennsylvania. Rick Saccone thanked President Trump as he exited the voting booth.
See below for a round-up of the sights and sounds of Election Day so far in Pennsylvania's 18th Congressional District.
5:42 P.M. - Both candidates urging last minute voters to get to the polls
With just two and a half hours left until polls close, both Republican Rick Saccone and Democrat Conor Lamb are taking to Twitter to urge those last minute voters to get out and vote before 8 P.M.
Turnout is expected to be key in the special election tonight, and both candidates are obviously aware they can't leave any votes on the table.
3:51 P.M. – Candidates project aura of optimism
ABC’s News "18 for 18" reporter Adam Kelsey, on the ground in the 18th Congressional District, reports that both campaigns have attempted to project an aura of optimism as voters head to the polls.
Democrat Conor Lamb’s campaign aides and volunteers are downright bubbly when you speak to them, clearly aware that their candidate has made major strides in this deep-red district, win or lose.
On the other side, GOP candidate Rick Saccone told ABC News on Monday that he was feeling confident.
As for voters?
Then there are the union members who want to save their jobs and think Lamb is the candidate who will look out for them because Saccone has defended "right to work" laws.
And there are union members who want to save their jobs and know that Saccone will back Trump's tariffs (although Lamb supports them too) and the "America first" agenda to do it.
3:38 P.M. – Analysis: Democrat needs mathematical trifecta to win
An analysis by ABC News' MaryAlice Parks found that Democrat Conor Lamb needs to pull off an electorate trifecta to win. The math would have to include dramatically boosting Democrat turnout over what it normally would be in a midterm or special election, convincing moderate or swing voters to go blue and hoping for depressed Republican turnout. For more, read here.
6:00 A.M. – The Note: What’s at stake in Pennsylvania’s special election
It will be up to a deep-red House district today to provide a blueprint – or deliver yet more Democratic blues.
Democrats are benefiting from some fortuitous casting with the 33-year-old, telegenic Marine reservist who is running in the special election in southwestern Pennsylvania. (Given state Democratic rules around this special, Lamb was nominated without having to navigate a primary.)
A Democratic win would be remarkable in district Trump carried by nearly 20 points. But it would also be remarkable for a Democrat to win while sounding and acting so unlike the national Democratic lineup he hopes to join. Read more here.
WATCH LIVE TONIGHT: You can watch livestreaming coverage of Pennsylvania’s special election as part of ABC News’ “18 for 18” coverage of the midterm elections starting at 8 p.m. ET on ABCNews.com or on the ABC News app available on the Apple App Store, GooglePlay Store, Apple TV App Store, and Roku Channel Store. Don’t forget to sign up for Midterm Elections Alerts to get more coverage of this year’s election season as par from our powerhouse politics team.
ABC News' MaryAlice Parks and Soorin Kim contributed to this report.