GOLDEN, Colo. -- Donald Trump launched a new round of claims about voter fraud Saturday at a rally here in Colorado, a state that is conducting its presidential election by mail for the first time.

“I have real problems with ballots being sent, does that make sense?” Trump said.

The Republican presidential nominee seemed to allege, without presenting specific evidence, that those collecting ballots might discard the ones marked with candidates they did not like: “Like people say, ‘Oh, here’s a ballot, bing, here’s another ballot, throw it away, throw. Oh, here’s one I like, we’ll keep that one. I have real problems, so get your ballots in. We’re trying to have some pretty good supervision out there. We have a lot of people watching you people that collect the ballots.”

Voters in Colorado are receiving their ballots by mail this year. They can send them in or give them up to vote in person instead.

Colorado Secretary of State Wayne Williams (R) issued a statement last month that said: “While vote fraud is rare, it does indeed occur. Our office is working to ensure all such incidents are prosecuted and that laws and rules are adjusted to make vote fraud as difficult as possible.”

His statement came in response to a local report about potentially fraudulent votes.

Documented instances of voter fraud are extremely rare, studies show. An episode emerged this week when an Iowa woman who said she supports Trump was arrested and charged with voting twice. She told Iowa Public Radio that she voted for Trump the first time but was afraid her vote would be switched to a vote for Clinton.

Trump has routinely argued the system is “rigged” against him, and he has repeatedly refused to commit to accepting the outcome of the election if he loses. He has raised concerns about fraud in the election process but has presented no concrete evidence for his claims. Officials in both parties have criticized Trump for raising alarm bells without proof.

“We’re winning Florida. I think we’re winning Colorado if it’s a straight-up system. I mean get those ballots in. I don’t love the concept of ballots. I don’t love it,” Trump said toward the end of his remarks Saturday, appearing to argue against a mail-in voting system. “Get those ballots in and follow your ballot.”