The Obama campaign made a big effort this week to encourage debate-and-vote parties like the one at Ms. Marshall’s home here in Larimer County, another pivotal county with a lot of mail-balloting.

Image Ms. York did not have a ballot, but kept notes to use later. Credit... Carmel Zucker for The New York Times

Mail-in voting has put down its deepest roots in Oregon, Washington and California, but election experts say the significance of Colorado’s mail-in voting this year has been amplified because the state is one of the few tossups left on the electoral map.

Previously, voting by mail in Colorado has been most common in rural areas, where distances make a trip to the polls problematic and Republican voters usually dominate. But the clerk and recorder for Weld County, Steve Moreno, said the Obama campaign, in particular, had embraced the idea of voting-by-mail this year and met with him about how to expand the numbers.

Ms. Marshall, the debate-and-vote party hostess, said she was voting by mail for the first time partly because the Obama campaign, for which she has volunteered, had pushed it hard as a tool to increase turnout.

“I have the time to go wait in line at the polling place, but the person behind me in line maybe doesn’t,” said Ms. Marshall, a graduate student in ecology at Colorado State University. “They might have only half an hour to wait on line at lunch break, and if they can’t get through the line, they may not vote.”

Statewide, officials said, there are broader reasons for the spike in mail-in voting, a subset of so-called early voting, which also includes voting in advance at polling places. The reasons range from anxiety over the state’s voting machines after a scare involving their technical reliability this year, to a state ballot with more issues and referenda on it than any in 96 years. County clerks also pushed the shift with advertising and mailings, hoping to avoid the long lines that plagued polling places in 2004 and 2006.

“I like the fact that you can sit down and think about it,” said Jim Richardson, a 56-year-old retired school teacher and coach who lives in Greeley, about 40 minutes from Fort Collins, in Weld County.