"Well, we'll have to see what happens," the president told a news conference at the White House. "You know that."
Mr Trump also said he believed the election result could end up in the US Supreme Court, as he again cast doubt on postal voting.
More states are encouraging mail-in voting, citing the need to keep Americans safe from coronavirus.
Mr Trump was asked by a reporter on Wednesday evening if he would commit to a peaceful transfer of power "win, lose or draw" to Democrat Joe Biden. The president currently trails his challenger in national opinion polls with 41 days to go until the election.
"I've been complaining very strongly about the ballots," Mr Trump, a Republican, said. "And the ballots are a disaster."
When the journalist countered that "people are rioting", Mr Trump interjected: "Get rid of the ballots, and you'll have a very - you'll have a very peaceful - there won't be a transfer, frankly, there'll be a continuation."