Trump then told supporters that they can pick up a new ballot in person.
"They'll give you a ballot, a new ballot. They'll void your old ballot, they’ll give you a new ballot. And you can go out and make sure it gets in," Trump said.
He suggested that this could lead to voter fraud but did not appear to recommend that his supporters vote several times.
"In some places they probably do that four or five times. But we don't do that. But that's great," he said.
Trump also expressed skepticism over Colorado's mail-in ballot system during a rally there on Saturday.
"I have real problems with ballots being sent," he said. "People say, oh, here's a ballot, bing. Here's another ballot, throw it away. Oh, here's one I like, we'll keep that one."
Watch a clip from Trump's Sunday rally via CNN: