No need to go into all the inside baseball details, but former Governor Bill Owens made some bad decisions that split the party back in the 2004-2006 time frame.
Also, this was the election when Obama took Colorado based, in my opinion. on Bush fatigue.
Another star, besides Gardner, has been Scott Gessler who won the Secretary of State position. (He's running for governor in this election.)
To understand it all, it starts with Fred Barnes 2008 article on the Colorado Model. Here.
There's a quick read of how it happened from a 2010 Red State. Here.
The book mentioned is a good read, more in-depth, and shows how other states Democrats are working behind the scenes, here.
Colorado Progressive Democrats do have a good Get Out the Vote effort and it cannot be ignored. Colorado Republicans can win but only if we are united behind a specific candidate, which hasn't happened in some of our past elections. Democrats here work hard to depress the vote and split our tickets.