(Gray counties did not yet exist and thus I could not calculate the Partisan Voting Indexes (PVIs).)
Early in the 20th century Washington leaned Republican, with the western counties being more Republican than the eastern counties. In 1928 all the counties had Republican PVIs except timber and mining union-dominated Ferry County.
From the Great Depression until 1960 Washington leaned Democratic as Seattle, Tacoma, and other population centers turned blue. The Republican swing in the east resulted in Washington having a light red PVI in 1964. Being one of only two Western states (Hawaii the other) to vote for Hubert Humphrey, Washington would continue to have a swingy PVI from 1968 through the 1970s. Washington became slightly redder in 1980 because like the rest of the West except Hawaii, it never voted for Jimmy Carter (D).
In 1984, Washington returned to a Democratic PVI and would remain there to this day. In the 1990s, the divisions between east and west, with the Cascades as the dividing line, became more apparent. The east became mostly Republican outside Spokane and socially liberal Whitman County, and the west mostly Democratic outside of Lewis County. Lewis began trending Republican in 1968 and has remained a Republican county since then. Some of the counties along the Columbia River also began trending Republican in the 1990s. In 2000 Ralph Nader (G) overperformed, which made Whatcom and Skagit Counties look more Republican than they actually were.
In later elections King County (Seattle) became dark blue. Thurston County (Olympia) also trended Democratic. Democratic gains in areas like these outweighed Republican trends elsewhere, considering that Washington’s PVI became slightly bluer in the 2010s than the 2000s, when Washington was still seen as a swing state. The working-class counties of Grays Harbor and Pacific trended Republican like the demographically similar counties in coastal Oregon. Along the Columbia River, Cowlitz and Wahkiakum Counties trended Republican and voted that way in 2016. Clark County (Vancouver) also trended Republican though it barely went Democratic in 2016.
Here are the results in table form.
Here are the results in map form.