Thousands of voters in Greeley gathered to hear the Republican nominee.
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Trump said he was skeptical of the state's largely vote-by-mail process, in which voters can request a new ballot in person.
All registered voters in Colorado receive a ballot by mail.
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"You can go to University Center, they will give you a ballot, a new ballot," Trump said. "They'll void your old ballot, they will give you a new ballot. And you can go out and make sure it gets in."
It's unclear whether Trump was encouraging people who have already voted to do that again, which would constitute voter fraud.
"In some places, they probably do that four or five times, but we don't do that," he said.
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