Donald Trump speaks during a campaign rally at the Jefferson County Fairgrounds on Saturday, Oct. 29, 2016, in Golden.

(AP Photo/ Evan Vucci)

Republican presidential candidate Donald Trump tried to stoke doubts about the integrity of Colorado's vote-by-mail system at campaign stops in Golden and in Greeley.

Nearly 20 percent of Colorado voters have returned their ballots, with less than two weeks to go before Election Day.

Trump suggested that votes are selectively counted depending on the preferences of who's counting. But he offered no evidence that has ever happened.

In Colorado, County clerks won't tabulate how people have voted until November 8, but officials do keep track of returned ballots by party registration.

According to data published by the Colorado Secretary of State's office, almost 30,000 more Democrats have turned in their ballots than Republicans as of Thursday morning. That's a change from 2014, when Republicans had the lead in early voting.

Trump said that he has "real problems with ballots being sent" by mail. He described a purely fictional scene in which counters sort through ballots, saying: "Here's another ballot, throw it away. Oh here's one I like, we'll keep that one."

He said his campaign has "a lot of people watching you people that collect the ballots." And he's advising supporters to track their ballots to ensure they've been counted.

Trump has repeatedly raised fears about the integrity of the U.S. voting system. Experts say voter fraud is rare. And Colorado officials say, when it comes to this state, Trump's claims are without merit.

“While there are occasional instances of voter fraud, Colorado’s processes are very good at catching attempts to commit voter fraud. We are working to improve our processes and prosecute those who break the law,” Republican Secretary of State Wayne Williams said recently.