Colorado was clearly a lean Republican state at that time. But it does appear back then, Colorado was very elastic and extremely responsive to national trends and has had a two-party tradition. The Democrats won a huge victory statewide in 1958. Then the Republicans came back in 1960 and 1962. And with the Democratic trend in 1964, Colorado went strongly for Johnson. And it went back heavily Republican in 1966 and again in 1968.
Colorado in the 1960s seems similar to New Hampshire these days.
There is a political article from June 1965 about the 1964 election in Colorado--written by Conrad McBride, who was a long time professor at the University of Colorado. He also published an article on the 1966 election as well. It's under a pay wall, but you can "rent" the articles free for 2 weeks:
https://journals.sagepub.com/doi/abs/10.1177/1065912965018002-205?journalCode=prqa
