A Democratic underdog is close to pulling out a huge upset in the nation's biggest election

• Democrat Conor Lamb is close to pulling off a remarkable victory in Pennsylvania's 18th Congressional District on Tuesday.
• President Donald Trump won the district by 20 points in 2016.
• Right now, the race is too close to call.

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Democrat Conor Lamb late Tuesday moved close to pulling off an improbable win, keeping the Democratic momentum going on Tuesday as he led Republican Rick Saccone in the special election for Pennsylvania's 18th Congressional District.

With more than 95% of precincts reporting, Lamb held a lead of less than 1 percentage point over Saccone.

Polls heading into Tuesday showed a tight race in a district President Donald Trump won by 20 points in 2016, with some showing Lamb, a former Marine and federal prosecutor, ahead significantly. The seat became open after longtime Republican Rep. Tim Murphy resigned from office late last year following an abortion-related scandal.

Republicans went into the day blasting Saccone, a state legislator and Air Force veteran, for running a subpar campaign in a race they increasingly expected to close. Trump still enjoys decent approval ratings in the district and attempted to push Saccone over the top with a weekend visit.

"Conor Lamb is gonna win," a national Republican close to the race conceded to Business Insider hours prior to polls closing. "I think it's been a race, since day one, about candidate disparity."

Another Republican strategist close to the race cited the "disparity between the candidates having an outsized impact."

"Conor Lamb is exciting, he does break the mold a little bit," the strategist said. "And he does sound like the kind of Democrat that your typical voter in that sort of district could support. Saccone, unfortunately just has never been able to excite people in the district. His shortcomings were noticed by a handful of strategists early on."

Lamb has dominated the fundraising battle against Saccone in the special election for Pennsylvania's 18th Congressional District, which Republicans highlighted as a significant problem for their candidate.

Outside groups had sought to make up the fundraising difference by injecting money and support into the race. The Congressional Leadership Fund, for example, provided 50 door knockers and spent more than $3.5 million on the election effort. The race featured more than $12 million in spending on both sides, with the airwaves flooded with an inordinate number of ads.

Lamb's performance is of huge concern to Republicans heading into the 2018 midterms, which increasingly look as if they will be a challenging landscape for GOP House and Senate candidates. Traditionally, the first midterm elections under a new president go for the party out of the White House.

"It should serve as a wake-up call to everybody," one Republican strategist said of the race.

Lamb ran as a centrist Democrat, embracing gun rights, expressing opposition to abortion personally but supporting abortion rights, and saying that he supports a middle class tax cut though he vehemently opposes the Republican tax plan. Saccone pitched himself as "Trump before Trump" and sought to tie himself closely to the president and his agenda, which remain decently popular in the district.

A number of policies were on the ballot as well, by proxy. This was the first major election since Republicans passed their major tax overhaul. At the same time, Trump reportedly pitched his steel and aluminum tariffs with an eye on the race, hoping they could help prop up Saccone. But polling showed the tariffs did little to swing votes.

By a weird quirk, both Lamb and Saccone could be members of Congress by January 2019. Lamb and Saccone are both likely to run in different districts if they choose to seek office again because Pennsylvania's congressional map will be redrawn before this fall's races.