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Boulder County will open vote centers on Monday where voters can cast their ballot in person, drop off their ballot or register to vote.
Counties across Colorado's Front Range are working to curb rising COVID-19 cases.
Ballots were mailed to voters last week and already 436,443 have been returned.
A major Democratic group on Friday pulled its last remaining ads from Colorado's closely watched U.S. Senate race, a sign that the party thinks its nominee has the crucial race in the bag.
The drop off ballot boxes themselves are bolted to the ground and have protections in place to keep ballots safe for the short time they are locked in.
Colorado's U.S. Senate race is one of the most highly anticipated in the nation as it could play a key role in the balance of power. Incumbent Republican Cory Gardner is fighting to keep his seat against former governor, Democrat John Hickenlooper.
Colorado state and local leaders said voter intimidation will not be tolerated during the election and they have measures in place to prevent it.
Sen. Michael Bennet dropped off his ballot, saying he hopes Colorado has the highest turnout in the nation for this election.
Cory Gardner is considered one of the most vulnerable Republican senators in the country because Colorado has turned sharply Democratic since Trump's election.
Denver Elections opened the McNichols Building on Monday. The building will be the city's main voter service office and polling center.
All registered voters in Colorado will receive a ballot in the mail. That's about 3 million active voters in the state.
A political "car rally" featuring Doug Emhoff, husband of Democratic vice presidential nominee Kamala Harris, was held in a parking lot of Denver East High School.
Colorado law determines that ballots can be sent 25 days in advance of Election Day. That date is Friday.
Colorado has more voters between the ages of 18 and 29 registered to vote than ever for this election.
Colorado Secretary of State Jena Griswold announced a new program to fix signature discrepancies during the November election.
