Republican Rick Saccone has conceded defeat in a closely watched election in Pennsylvania, more than a week after the end of a remarkable race that has shaken Republican confidence ahead of November mid-term elections.

Democrat Conor Lamb, 33, claimed the seat by about 750 votes in a Republican-held district that President Donald Trump won by almost 20 percentage points just 16 months ago.

Mr Lamb, who struck a moderate tone during the race and was backed by the district’s influential labour unions, beat Mr Saccone, a state legislator who had compiled one of the most conservative voting records in Pennsylvania’s legislature.

The winner also benefited from what Pittsburgh-area Democrats called the most energised electorate they had seen, driven by an anti-Trump fervour.

In a statement, Mr Saccone said that, while fewer than 800 votes separated them, the people of the 18th District “deserve to have a voice representing them in Congress”.

Republicans had considered seeking a recount request or a lawsuit to contest the result, even as vote-counting over the past week continued to extend Mr Lamb’s lead.

Officials from the National Republican Congressional Committee and state Republican Party said on Wednesday night that they were dropping any such plans in light of Mr Saccone’s concession.

That means Mr Lamb could be sworn in after April 2, when the last of the four counties in the district expects to finish certifying the result.

Rick Saccone had one of the most conservative voting records in Pennsylvania's Legislature (Keith Srakocic/AP)
Rick Saccone had one of the most conservative voting records in Pennsylvania’s legislature (Keith Srakocic/AP)

The seat became open in October when eight-term Republican representative Tim Murphy resigned amid a scandal in which it emerged the anti-abortion lawmaker had asked a woman with whom he was having an extramarital affair to have an abortion when he thought she might be pregnant.

Locally, the outcome has a short-term effect.

The district is dissolving under a court-ordered redrawing of Pennsylvania’s 18 congressional districts, leaving Mr Lamb to seek the nomination to challenge three-term Republican representative
Keith Rothfus in a newly drawn district west of Pittsburgh.

Mr Saccone is running in a new, more Republican district in Pennsylvania’s south-west corner that retains much of the territory he and Mr Lamb campaigned in for the last four months.

Nationally, Mr Lamb’s win highlights Democratic potential and Republican vulnerabilities ahead of the autumn campaign with the Republicans trying to defend their majorities in Congress.

In the race, Mr Lamb downplayed opposition to Mr Trump, who remains more popular in the district than nationally.

Mr Lamb opposed sweeping new gun regulations and supported Mr Trump’s steel tariffs, but he is a fierce critic of Republican tax cuts and their economic and health care policies.

Mr Saccone, 60, adhered to Republican orthodoxy and openly embraced the president, who tweeted numerous times on his behalf and campaigned in person twice, including a raucous rally the Saturday before voters cast ballots.