WASHINGTON (Reuters) - The Republican candidate in a close U.S. House of Representatives special election on March 13 in Pennsylvania conceded to his Democratic opponent on Wednesday, cementing a Democratic upset in a long-time Republican area that President Donald Trump handily won in 2016.

In an ominous sign for Trumps Republicans eight months before national midterm elections, moderate Democrat Conor Lamb led conservative Rick Saccone by a fraction of a percentage point in the race for the southwestern Pennsylvania seat.

Trump won the district by almost 20 points in the presidential election. He campaigned for Saccone, who started the race well ahead of Lamb.

Just got off the phone with my opponent, @RickSaccone4PA, who congratulated me & graciously conceded last Tuesdays election, Lamb said on Twitter.

An official at Saccones campaign confirmed the candidate had conceded.

The earliest the final election result could be certified is March 26, but the final tally could be unknown for weeks.

Lamb led Saccone by 627 votes unofficially, state returns showed last week; Lamb had 49.8 percent of the vote and Saccone 49.6 percent.

House Republicans had called the race unique, noting that Lamb, 33, a Marine Corps veteran, had distanced himself from his partys leaders and staked out positions to the right of many Democrats .

The patchwork of small towns, farms and Pittsburgh suburbs that make up Pennsylvanias 18th congressional district has been so staunchly Republican that Democrats did not field candidates in the previous two House elections.

Come November, the district will cease to exist because boundaries have been redrawn. Both Lamb and Saccone are expected to run again, though in different districts.

The election, held to replace a Republican who resigned amid a scandal last year, was the latest forceful electoral showing for Democrats, who also won a governors race in Virginia and scored a U.S. Senate upset in conservative Alabama.

Lambs strong showing could buoy Democrats nationally as they seek to win control of the House from Republicans in the November elections. Democrats see 118 Republican-held districts in play. If they flip 24 seats, they could reclaim a House majority.

The Lamb win vindicates a strategy Democrats are using in some races to enlist candidates whose positions and ideologies are well suited to the district even while conflicting in significant ways with the positions of the Democratic leadership in Washington.

(Reporting by Eric Walsh; additional reporting by David Morgan; editing by Toni Reinhold and Grant McCool)