A slim majority of Virginia voters backed new congressional maps that could allow Democrats to gain as many as four House seats ahead of November's midterm elections.

Virginia backs new maps by a narrow margin

Virginia voters approved a redistricting measure by 51.4 percent in favour to 48.5 percent against, reshaping the state's congressional boundaries in a way that strengthens Democratic prospects for the midterms.

The new maps could allow Democrats to flip as many as four House seats, making it easier for the party to take control of the US House of Representatives in November.

President Donald Trump called the vote "rigged" following the result.

Part of a wider national fight

Virginia's decision is one move in a broad, tit-for-tat redistricting contest playing out across the country ahead of the midterm elections. Experts have warned that the unprecedented pace of map redrawing risks a surge in gerrymandering that could ultimately harm voters.

Other states have moved in the opposite direction. Florida lawmakers approved new voting maps drawn to favour Republicans, and the US Supreme Court reinstated a Republican-favoured Texas electoral map that the Trump administration backed, with analysts saying it could flip key districts to Republicans.

The Supreme Court also voided Louisiana's existing voting map in a ruling widely seen as a significant blow to the Voting Rights Act. Critics said the decision could dilute the voting power of minority groups. Trump described the Louisiana ruling as a "big win" and said Tennessee would be next to redraw its maps under the authority the ruling provides.