In the Truth Social post, Trump made vague claims about polling, voting "dumps", and "machines" without providing specific evidence of error or wrongdoing.

He has previously attempted to cast doubts on the integrity of elections he has lost, although he himself was not on the ballot in the midterms.

"We would have won easily if I was on the ballot," Trump told CBS News, the BBC's US partner, after it was confirmed his party had lost both the Senate and the House.

As it stands, Democrats have won 54 US Senate seats and 254 US House seats, giving them comfortable majorities in both chambers even as counting continues in some races.