(Gray counties had insufficient data to calculate Partisan Voting Indexes (PVIs)).
Texas was part of the Solid Democratic South in the early 20th century except when it voted for Herbert Hoover (R) in 1928, though it had some Republican strength in the Hill Country around San Antonio and Austin. This area, especially Gillespie and Kendall Counties, was home to German Americans that resisted secession. This area was self-sufficient and sold some of its surplus food to the Union Army.
Texas voted for Dwight D. Eisenhower (R) both times and began its hard swing away from the Democrats in 1956. Governor Allan Shivers (D) clashed with President Harry Truman (D) over the federal government claiming the tidelands, which were stipulated to be Texas territory when the state was admitted in 1845. In 1952 Eisenhower pledged to restore tideland control to the states, and Shivers worked to elect him.
Voting for John F. Kennedy (D) in 1960, native son Lyndon B. Johnson (D) in 1964 and Hubert Humphrey (D) in 1968 (the only Southern state to do so), Texas maintained a light blue leaning through the 1960s. Further reddening of the suburbs pushed Texas over to a light red lean in the 1970s even though it voted for Jimmy Carter (D) in 1976. That would be the last time Texas voted Democratic for president between then and now.
With George H. W. Bush (R) on the ticket as vice president and then president, many rural counties trended Republican in the 1980s along with the suburbs, solidifying Texas’ status as a red state. This redness peaked when Bush’s son, Governor George W. Bush (R) was elected president. In 2004, the Democratic strength was mostly concentrated in the cities and most of the Mexican border counties, though the latter voted less Democratic in the 2000s than in the 1980s and 1990s because of the younger Bush’s courting of the Hispanic vote. This area swung back to mostly dark blue later.
Also, Travis County (Austin) and El Paso became considerably blue. Dallas County flipped to a blue Partisan Voting Index (PVI). Tarrant (Fort Worth), Denton, and Collin Counties became less dark red. Harris County (Houston and many suburbs) became light red. These trends resulted in Texas slowly returning to a Republican PVI of less than R+10 by 2008.
Many rural counties continued trending dark red, though Democratic trends in cities and suburbs moved the PVI to just under R+8 in 2016. Bexar (San Antonio) and Harris Counties flipped to Democratic PVIs and Fort Bend (Sugar Land) next to Harris almost did. Tarrant also became a lighter shade of red.
Here are the results in table form.
Here are the results in slide form.