Copper mining and cotton farming, and later tourism, dominated Arizona’s economy in the 1920s and 30s, which made the state more Democratic relative to the country. Democrats here had historically been known as “Goldwater Democrats” or “Pinto Democrats”, socially liberal and fiscally libertarian. Arizona began trending Republican after World War II with the migration of snowbirds from the Midwest and to a lesser extent the Northeast, with the Republicanism peaking in the Goldwater and Reagan years. Afterwards, the trend has been slightly Democratic.
Apache County trended the same way as the state before breaking away from that pattern in the 1970s as the local Native Americans, mostly Navajo, began voting Democratic in large numbers. Apache did vote for Reagan in 1980 due to Carter’s perceived weaknesses on issues specific to the West, especially water. Now Apache is a strongly Democratic county. It was the most Democratic county until Santa Cruz took over in 2012.
Cochise County was a few points less Republican than Arizona and largely followed the state until the 1990s, when it trended more Republican. Democrats have strongholds in liberal Bisbee and Hispanic-heavy Douglas, but they are outweighed by the conservative/military vote of Sierra Vista and Fort Huachuca.
Coconino County, home to the Grand Canyon and Lake Powell, was a Republican stronghold from the 1950s to the 1980s. It has trended more Democratic of late due to a large college student/employee population in Flagstaff and many federal employees at the U.S. Forest Service, U.S. Geological Survey and the Bureau of Indian Affairs.
Graham County was a strongly Democratic county early on, having voted for Cox in 1920 and Davis in 1924, before swinging right in the 1950s. Now, with a significant Mormon population (it was Romney’s best county in the 2008 primary), it battles with Mohave for most Republican county in the state.
Greenlee County was the most Democratic county in Arizona until 1996, voting for McGovern and Mondale, due to unionized miners from the Morenci copper mine. The breakup of the unions in the 1983 Great Arizona Copper Strike, and the diminishing way of life in a copper mining town led to Greenlee trending more Republican, having voted that way in every presidential election since 2000.
Gila County has also historically been Democratic, only going for the Republican in landslides such as 1956 and 1984, until 2000. Its trends have paralleled Greenlee, including a rightward turn in 2000.
La Paz County has been a consistently Republican-voting county since it was carved out of Yuma County in 1983.
Maricopa County, home to Phoenix, Tempe, Mesa, and many other cities, voted strongly Republican from the 1960s to the 1980s because of the migration of many Midwestern snowbirds who were very receptive to Goldwater, Nixon, Ford, and Reagan. The East Valley is home to white-collar tech workers, as well as heavily Mormon Mesa. North Phoenix has a considerable evangelical population, and the West Valley has a lot of blue-collar workers and retirement communities in places like Sun City. Democratic strongholds are in heavily Hispanic South Phoenix, downtown with African Americans, and college town Tempe. The growing minority population, as well as possible moderation among suburbanites in the East Valley, are contributing to Maricopa’s slow leftward trend since the 1990s, notwithstanding the McCain bump in 2008.
Mohave County, home to Lake Havasu and part of Grand Canyon National Park, saw a slight moderation from its usual strongly Republican voting patterns in the 1990s, due to the Grand Staircase National Monument resonating positively with local voters. Later, however, the county rapidly became more Republican and has remained conservative, being the only county to vote against Prop 100 in 2010.
Navajo County has leaned Republican because conservative white voters, a lot of them Mormon, turn out in greater numbers than the Native American voters.
Pima County, home to Tucson, has historically been a swing county, though of late has leaned more Democratic. The college vote from the University of Arizona is balanced out by the military vote from employees of Davis-Monthan Air Force Base. Pima was the only county to vote against the 2008 gay marriage ban.
Pinal County, between Maricopa and Pima, has been a bellwether county in Presidential races, voting for the winner from 1912 to 2004 except 1968 when it voted for Humphrey. Some suburbanization from Maricopa County has produced a Republican trend.
Santa Cruz County was a swing county for much of the post-war period until the 1990s. Now, the significant Hispanic population and concerns over immigration make Santa Cruz the most Democratic county in the state, having surpassed Apache in 2012.
Yavapai County, home to Prescott and most of Democratic-leaning Sedona, has remained a strongly Republican county due to a large evangelical population in Prescott and strong concern for the Second Amendment. Yavapai has voted Republican in every election since it was created, except for FDR’s 4 wins and Truman.
Yuma County voted more Democratic than the country from 1948 to 1960, though Goldwater in 1964 made the county turn sharply Republican and trend Republican through 2012. About 60% of Yuma’s population is Hispanic, and many of them are Democrats, though Trump carried the county by 1 point. Yuma’s economy is closely tied to those of the Mexican states of Sonora and Baja California.