The newest Colorado voter registration numbers, as of October 1 (via the Secretary of State’s office). The first number is active voters, the numbers in parentheses are total registered:

DEM: 799,981 (1,073,165) 74.5%
REP: 862,575 (1,079,411) 79.9%
UAF: 757,935 (1,105,522) 68.6%
ACTIVE: 2,437,772
INACTIVE: 845,083
TOTAL VOTERS: 3,282,855

From the numbers, it is clear Republicans hold an advantage over Democrats both in absolute terms (total registered party members) and designation as “active” by the Colorado Secretary of State. While unaffiliated voters outnumber both parties, constituting the largest bloc of voters in the state, they participate at a lower level, with far more “inactive” voters.

Total permanent mail-in ballot voter registration sits at 60.2 percent of the “active” voting population (the 12 largest counties are only slightly higher, at 61.5 percent absentee). Arapahoe, Douglas, and Jefferson counties–all key Denver suburbs–sit above 70 percent.

Rich Coolidge, spokesman for the Secretary of State’s office, clarified the “active”/”inactive” breakdown for Battle ‘10:

There are 2 kinds of inactive voters. Inactive — failed to vote, or those who did not vote in an even numbered year and failed to respond to a mailing asking if they wished to remain an active voter. These people will still get ballots and mailings. The other is inactive — undeliverable. These voters have had official mail returned as undeliverable. These people will not be mailed a mail ballot but can still vote at their polling place. Inactive voters are still considered registered voters. [emphasis added]

Most polling (in the U.S. Senate race, for example) focuses on some configuration of a “likely voter” model, which should not necessarily viewed as correlative with the state’s “active” voters. Certainly the propensity to vote remains highest with those voters already holding the “active” status, and those registered for absentee voting are even more likely to cast ballots. Mail-in balloting begins next week, and voter registration for the general election will close today.

Total Colorado registered voters from January 2004 to present (all numbers available at the SOS site).

Unaffiliated voters have increased in registration from 900,000 to over 1.1 million, adding 200,000 in just over five years. Democrats have added about 220,000 voters (adjusted after the 2006 election) in just over three years, temporarily overtaking Republicans for about 18 months following the 2008 election. Republicans have once again edged out Democrats for second largest voting bloc by the slimmest of margins. The temporary dip in unaffiliateds and bump in registered Rs and Ds can most easily be explained by registration for the August primary, as party affiliation is required for primary participation.