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 in close vote

Virginia voters have approved a redistricting measure that redraws the state's congressional boundaries in favour of Democrats, with the referendum passing 51.4 percent in favour to 48.5 percent against.

The new maps could allow Democrats to flip as many as four House seats, strengthening the party's bid to retake control of the US House of Representatives in November.

President Donald Trump called the vote "rigged" shortly after results were confirmed.

Part of a wider national fight

The Virginia result is one front in an accelerating national redistricting contest between Democrats and Republicans. Experts have warned that the unprecedented frequency of tit-for-tat map redrawing risks a surge in gerrymandering that could ultimately harm voters regardless of party.

Elsewhere, the battle has moved in the opposite direction. Florida lawmakers approved new voting maps designed to boost Republican seats, making the state the latest to act in what has become a running national fight over which party controls the House.

The US Supreme Court has also shaped the landscape. The court reinstated a Republican-favoured Texas electoral map that backers say could flip key districts to Republicans. A separate Supreme Court ruling limited the use of race in drawing electoral boundaries, a decision that critics say will make it harder to challenge maps that dilute the voting power of racial minorities. That ruling voided Louisiana's existing voting map and, according to Trump, opens the door for Tennessee to redistrict next.