New Deal public works, most especially such developments as Grand Coulee Dam and the water projects on the Sacramento and San Joaquin, seem to me the most likely reason. With drought having been persistent in the West throughout most of the 1920s (1923/1924, 1928/1929, and 1930/1931 were very bad “rain years”, as was 1933/1934 in the south with a remarkably warm winter) people were desperate for water security in the drier parts and for infrastructure in the wetter areas.
FDR’s success was replicated all over the West: west of the Continental Divide Alf Landon won only rock-ribbed Kane County, Utah, along with Clark County, Idaho and Rio Blanco County, Colorado. This despite the fact that on the eastern slopes Landon – aided by his support for Prohibition – took many counties won by FDR in 1932.
Before 1932, hostility towards the Democratic Party was very widespread in Western Oregon and Western Washington due to its association with Catholicism and the Confederacy in what was even them the most secular, “Scandinavian” region of the country. In 1928, Washington was thefourth most Republican state in the nation and California the eighth most Republican, although Al Smith did carry four presumably Catholic slope counties (Amador, El Dorado, Placer and Plumas, of which the latter pair were very solid Republican from 1860 to 1908). This hostility – though chipped at mildly by Bryan and Wilson – was really only broken by FDR in the desperation of drought, alternating very mild and cold winters, and severe economic depression. Then, the mountainous West offered opportunities for public works development (chiefly dams) largely absent in the East.
San Francisco went for Smith, Wilson, and Cleveland (in the case of 1892, that was the flipping point)
