Voting by mail starts in a little more than a week.
History proves it is popular.
"In 2008 about 63 percent of every voter in Colorado voted by mail. And in 2010, 69 percent so its gone up a little bit," said Secretary of State Scott Gessler.
With so many people potentially voting before election day itself, the campaigns are rushing to reinforce their messages.
"They're going to start making decisions. That means the campaigns really have to get to them to persuade them," said University of Denver political scientist Peter Hanson.
The Obama and Romney campaigns are on aggressive timetables..
Campaign supporters are making appearances across the state all weekend.
"They're going to be putting out their best ads and best arguments out right now. It used to be that you might save those until the week before the election. I just think that strategies changed. We're going to see them make an all out effort starting in the next week or so," said Hanson.
Another looming deadline is October 22. It is the start of early voting in Colorado. Voters can show up at the polls in person to avoid voting on election day itself.