Canonsburg:  Republicans have sounded alarm bells after Democrats claimed victory in a Pennsylvania congressional election seen as a referendum on US President Donald Trump's performance.

Conor Lamb, a Democrat and former Marine, scored a razor-thin upset in a special House election in southwestern Pennsylvania after a few thousand absentee ballots cemented the result.

The Republican candidate, Rick Saccone, may still contest the outcome, but Lamb's 627-vote lead on Wednesday afternoon appeared insurmountable, given that the four counties in Pennsylvania's 18th district have about 500 provisional, military and other absentee ballots left to count, election officials said.

That slim margin — out of almost 230,000 ballots cast in a district that Trump carried by nearly 20 percentage points in 2016 — nonetheless upended the political landscape ahead of November's midterm elections. It also emboldened Democrats to run maverick campaigns even in deep-red areas where Republicans remain bedeviled by Trump's unpopularity.

The election is historically seen as an easy win for Republicans. Trump actively campaigned for Saccone, who started the race ahead of Lamb in a district Trump won by almost 20 points in the 2016 presidential election.