As of 2008, the number of eligible voters has grown with Colorado's population.
The estimated voting aged population in Colorado at the time of this November's election was 3,770,624. The source I linked suggests that 92.5% of voting aged Coloradans were U.S. citizens in 2008. Thus, about 282,797 people in Colorado are ineligible to vote because they are not citizens (note also that many non-citizens in Colorado are legal immigrants with appropriate resident visas). Of U.S. citizens, 22,481 people were ineligible to vote because they were incarcerated in prison for felonies, as were 8,196 people who were ineglible to vote because they were on parole for felonies. Thus, there were a total of 30,677 ineligible felons in Colorado. On the other hand, there were 73,854 overseas persons who were eligible to vote in the U.S. (mostly military personnel who are deployed, missionaries and their families, families of business people abroad, and foreign students).
Thus, there were 3,529,620 people eligible to vote in Colorado on election day.
The biggest barrier for many of these people was voter registration. In this election in Colorado there were 3,203,583 registered voters in the state, of whom 2,582,189 were "active registered voters." Inactive registered voters haven't voted in recent elections are are often people who have moved (or died) without updating their voter registration information.
The number of registered voters is 90.8% of the number of eligible voters in the state. The number of active registered voters in the state is 73.2% of the number of elgibile voters in the state. Probably at least half of eligible voters who do not vote are not eligible to vote on election day because they are not properly registered to vote.
While final numbers are working their way in, we are approaching a good estimate of turnout in this election in Colorado:
[A]s of Thursday, the secretary of state reported that 206,566 people voted on Tuesday, though the number will increase when everything is finally counted. Still, the total is expected to fall far short of the 1 million or more people expected on Election Day.
The statewide unofficial vote count is largely finished, though the final numbers are still in flux. Election workers still have to pore through problem ballots, such as mail ballots that are missing signatures, and provisional ballots cast by people whose voter registration is in question.
But the latest numbers indicate that roughly 2.25 million people voted - by mail, at early polling sites or on Election Day. Assuming that the same number of provisional ballots are validated this year as in 2004, that would take the number of voters to about 2.29 million.
That's a jump over the 2.15 million people who voted in 2004, but not the flood of voters that many elections officials predicted.
It also represents little change in percentage of voters who turned out. This year saw a slight increase in turnout considered against the number of registered voters but a slight decrease from 2004 when considered against "active" voters - those who cast ballots in the last even-year election, in this case 2006.
Turnout as a percentage of voting age population was about 60.7%. About 64.9% of those eligible to vote in this year's November election in Colorado did so. Turnout was 71.6% of registered voters, and 88.8% of active registered voters.
But, even turnout doesn't tell the whole story. Many voters abstain from voting on every matter presented to them on the ballot. Of those who voted for President (the item with the greatest voter participation):
* 2.8% did not vote for U.S. Senator
* 3.8% did not vote on Amendment 48 (the state Amendment with the least drop off)
* 11.9% did not vote on Amendment M (the state Amendment with the most drop off)
* 18.7% did not vote for a member of Congress (although drop off varied by Congressional District).
* In judicial retention races for the statewide offices of Colorado Supreme Court Justice and Colorado Court of Appeals Justice, the percent not voting ranges from 26.0% to 27.6%. (Notably, all appellate judges did receive an affirmative "yes" vote from a majority of those voting in the election, although the margin was only about 51% of those voting for the justices and judges receiving the least votes in absolute terms).
In the three C.U. Regent races, 20.9% to 44.5% of those who voted in the corresponding Congressional race did not cast a vote for a C.U. Regent candidate. In the two State School Board races, 7.4% to 26.2% of those who cast a vote in the corresponding Congressional race did not cast a vote for a State Board of Education candidate. Keep in mind that these drop offs are on top of the substantial drop off from the Presidential race to the Congressional races.