After consistently voting more Republican than the country as a whole beginning in the 1920s, Colorado began its trend to the Democrats in 2004, flipping to a Democratic PVI in 2012. Even in 2016, Colorado voted slightly more Democratic due to Democrats’ gains coming mostly from the fast-growing suburban areas around Denver and Republicans’ gains coming mostly from slower-growing or no-growth parts of the state.
Coming off the Wilson years, in which the Democrat won big in the western half of the state, Colorado shifted sharply Republican, with the eastern plains counties and some northern counties leading the way. Colorado’s first ski resorts were established in the 1930s, and the counties where those ski resorts were located, such as Pitkin (Aspen) and San Miguel (Telluride), mostly voted Democratic ever since. (Breckenridge also likely voted Democratic, but Summit County remained Republican until the 1990s.)
In the 1940s, Republicans strengthened their hold on the eastern plains counties and the western counties outside of Grand Junction and the ski resorts, swinging the state back to the Republicans. Democrats at this time strengthened their hold on the San Luis Valley in southern Colorado, including Pueblo.
After World War II, Colorado became a little less Republican as Democrats made gains in the rapidly growing Denver metropolitan area, especially the city of Denver and Adams County. Except for the Reagan elections, Adams County has had a Democratic PVI since 1952. Other populous counties such as Arapahoe, Jefferson (both in the Denver area) and El Paso (Colorado Springs) voted more Republican than Denver and Adams voted Democratic, keeping Colorado in the Republican column from 1952 to 1988 (except 1964). Republicans were also helped later in the 1980s by Douglas County, in exurban Denver, which was rapidly growing.
Going into the 1990s and early 2000s, Democrats consolidated the college vote in Boulder and Fort Collins, and made gains in Arapahoe and Jefferson Counties. However, Colorado stayed in the Republican column (except 1992, possibly due to Perot) because counties in the east and west, as well as rapidly growing Douglas County and military-heavy/religious El Paso County, became even more Republican, and the San Luis Valley counties became less Democratic.
While Bush won Colorado in 2004, Democrats made further gains in the Denver area, which probably helped them win both houses of the state legislature. Bush won Arapahoe, Jefferson, and Broomfield 51-46, roughly the same margin he won statewide.
In the next election, Obama won those three counties by about 54-45, similar to his margin statewide. Increasing racial diversity in Aurora helped move Arapahoe County to the left of the state, while Jefferson and Broomfield voted similarly to the state in 2012. El Paso and Douglas remained solidly Republican, though trended slowly Democratic in recent elections. The college vote in Fort Collins moved Larimer into the Democratic column as well. The counties in the center-west also trended Democratic with the rise of ecotourism drawing many liberals to the mountains.
The western counties further from the mountains, anchored by Mesa County (Grand Junction), remained stable in the last few elections. Except for San Miguel, the rural, mostly white region is firmly in the Republican column.
The plains counties in Eastern Colorado culturally have more in common with Kansas and thus became rock-solid Republican. Weld County remained stable because the Republican vote in areas with a significant extraction industry was balanced by the increasing Hispanic population in Greeley. A 2013 state ballot measure that called for the secession of several northeastern counties passed in five counties in the area: Cheyenne, Kit Carson, Phillips, Washington, and Yuma. Even in the six counties that voted against secession, over 40% of voters voted “Yes” in Elbert, Lincoln, Logan, Moffat, Sedgwick, and Weld.
In the former Democratic stronghold of the San Luis Valley, home to blue-collar workers and Hispanics that settled in the area centuries ago when it was part of Spain, Republicans made significant gains. The area began trending Republican around 2000, with only Saguache (near the Rio Grande National Forest), Alamosa (commercial/college county), and Costilla (heavily Hispanic, about 2/3) remaining in the Democratic column. Even these 3 counties saw significant shifts toward Republicans.
Here are the PVIs for the state and each county in table format.
Here are visuals of the Colorado PVIs statewide and by county.