Democrat Conor Lamb narrowly led Republican Rick Saccone with most precincts reporting in a special U.S. House election in Pennsylvania Tuesday night, but the race was still officially too close to call.

With all but 22 precincts reporting, Lamb held a razor-thin 918-vote lead over Saccone as he rode a wave of Democratic enthusiasm in a district that President Trump won by 20 points a mere 16 months ago.

Lamb, a 33-year-old Marine veteran and former federal prosecutor, ran up big margins against Saccone in wealthy Allegheny County and was holding his own in GOP-leaning Westmoreland, Washington and Greene counties.

He’s trying to look like a moderate Democrat or look like a Republican but we know he’s not.

- GOP House candidate Rick Saccone on opponent Conor Lamb

Pennsylvania Secretary of State Wanda Murren told Fox News the race would not have a mandatory recount. Three voters in each precinct must petition for a recount and petitions must be filed five days after each county completes its tally.

Lamb insisted that Trump was not the main issue in the race. But the closeness of the race is another setback for the president following Democratic Sen. Doug Jones' victory in December's Alabama special election. The result expected to shake Republican self-assurance that their new tax law is an omnipotent offense and defense in their midterm matchups and raise Democratic hopes of taking back the House in November's midterm elections.

“We were executing a plan that we came up with a long time ago that had nothing to do with the president,” Lamb told reporters after voting Tuesday morning. “This is a local race … I don’t think it has anything to do with the president.”

By contrast, Saccone had vowed that he would be Trump's 'wingman,' telling Fox Business Network's 'Mornings with Maria' that the president 'needs some help down there [in Washington].'

The president visited the district twice to campaign for Saccone, once in January and again on Saturday night in a rollicking rally that recalled Trump’s own 2016 campaign.

This is a local race ... I don't think it has anything to do with the president.

- Conor Lamb, Democratic House candidate

“Do me a favor, get out on Tuesday,” Trump said immediately after greeting the crowd. “Vote for Rick Saccone, and we can leave right now.” Vice President Mike Pence, Ivanka Trump and counselor to the president Kellyanne Conway also visited the district.

But Saccone struggled to raise money and stir the same passions that helped Trump sweep the industrial Midwest on his way to the White House. According to Federal Election Commission records, Lamb raised more than $3.3 million since the start of 2018, compared to $703,000 for Saccone.

Meanwhile, national groups aligned with Trump, House Speaker Paul Ryan, R-Wis., and other Washington Republicans poured roughly $9 million on the race, filling airwaves and social media with depictions of Lamb as little more than a lemming for House Minority Leader Nancy Pelosi, D-Calif.

Lamb countered with an ad calling it all 'a big lie' since he'd already declared he wouldn't support Pelosi as floor leader, much less a return to the speaker's rostrum. He's added his opposition to major new gun restrictions — though he backs expanded background checks — and declared himself personally opposed to abortion, despite his support for its legality.

By contrast, Lamb has hammered the new GOP tax law as a giveaway to corporations at the certain future expense of Social Security, Medicare and the nation's fiscal security. Lamb has also embraced unions, highlighting Saccone's anti-labor record at the statehouse — a noticeable deviation from Murphy's status as a union-friendly Republican. The AFL-CIO counts 87,000 votes from union households — around a fifth of the electorate.

In a bid to lock up that key voting bloc, Democrats called in former Vice President Joe Biden.

“You said you want your piece of the sidewalk,” Biden, a potential 2020 presidential candidate, told a group of union workers last week. ”Hell, you own the sidewalk.” Biden has also said that Lamb reminds him of his late son, Beau, an Iraq War veteran and former Delaware attorney general who died of brain cancer in 2015 at the age of 46.

Pennsylvania's 18th congressional district, which stretches from the affluent Pittsburgh suburbs into deep Pennsylvania steel and coal country, had been held by Republican Tim Murphy since 2003. But Murphy was forced to resign in October amid revelations of an extramarital affair in which he urged his lover to get an abortion when they thought she was pregnant.

Lamb and Saccone could face off again in November – though they may not meet in the same district. In January, the Pennsylvania Supreme Court ruled that the state’s congressional district boundaries were unfairly gerrymandered to aid Republicans.

The Democrat-controlled court has drawn a new map that puts Saccone and Lamb’s homes in separate districts. However, the matter is now in the hands of a three-judge federal panel, which is considering an appeal by Republican lawmakers.

Fox News' Bret Baier,’s Joseph Weber and The Associated Press contributed to this report.