Assuming Kyrsten Sinema is the winner of the U.S. Senate race in Arizona, the Republican Party will have won only two U.S. Senate elections in 2018 in the western states. “Western states” means these thirteen states: Alaska, Arizona, California, Colorado, Hawaii, Idaho, Montana, Nevada, New Mexico, Oregon, Utah, Washington, and Wyoming.
At the beginning of the United States Constitution, the Senate seats were divided into three classes. The seats to be elected in 1790 were called the Class One seats. Class One seats were the seats up in 2018. There are nine western seats in Class One.
The two seats that went Republican in the west this year are Utah and Wyoming. There are four western states that don’t have a Class One seat: Alaska, Colorado, Idaho, and Oregon.
Here is the number of seats in the west that the Republicans won in past Class One election years, since the beginning of popular election of Senators: 1916 two; 1922 one; 1928 two; 1934 two; 1940 one; 1946 five; 1952 five; 1958 one; 1964 three; 1970 two; 1976 four; 1982 four; 1988 five; 1994 five; 2000 five; 2006 four; 2012 four.
In the United States as a whole, assuming Republicans won the Florida seat and Democrats won the Arizona seat, in 2018 Republicans elected ten Senators out of the 33 regular elections. Republicans won four seats in the midwest and four in the south. Class One seats are notable for having fewer southern seats than the other two classes.