US President Donald Trump has claimed without evidence that cheating took place in an election in Virginia that could help Democrats win control of the US House of Representatives in the midterm elections this year.

Virginians on Tuesday voted to redraw the state's congressional map, which could help Democrats win four House seats currently held by Republicans.

The state is the latest in a national redistricting arms race that began after Trump urged conservative states to re-examine voting maps to help Republicans keep their congressional majority.

"A RIGGED ELECTION TOOK PLACE LAST NIGHT," Trump posted on social media, echoing false claims he has made about the 2020 presidential race.

"All day long Republicans were winning, the Spirit was unbelievable, until the very end when, of course, there was a massive 'Mail In Ballot Drop!'"