Beginning at the southern edge of the Portland metropolitan area and extending south into farm and timber country, the Mid Willamette Valley's biggest city is one that cannot be ignored in state politics: Salem, Oregon's capitol. I'm identifying this region in the simplest way possible, as Marion, Polk, and Yamhill counties, though some may include Linn and Benton counties in the region, and perhaps the southwestern edge of Clackamas County.
French Prairie in the north end of Marion County is a place of historic importance to Oregon. Champoeg, near the border with Yamhill County, was the site of the formation of the first provision government of the Oregon Country. The region has other French placenames from the areas early white settlers: Gervais, St. Paul, Butteville. It is also a region close to my heart as I grew up in the communities just on the other side of the Clackamas County line. It tends to be a religious area with at least two Christian colleges (George Fox University and Corban College), and several religious settlements such as that at the Mt. Angel Abbey, which was founded by Benedictine Monks, and Aurora, which was founded as a religious commune. There is a thriving community of Russian Old Believers and other groups which sought refuge from the former Soviet Union as well.
But the Mid Valley is much more than just that.
The Mid Willamette Valley is home to almost 13% of the state's population, but currently has only one relatively prominent politician. The most prominent politician from the region is Senate President Peter Courtney (D-Salem). In the recent past, the most prominent politician from the area was Mark Hatfield, who was born in Dallas in Polk County, and lived in Salem. Previously the region had three members of congress, all of whom once held the 5th congressional district: Jim Bunn (R-McMinnville) who lost in 1996 and failed in a comeback attempt at the state house level in 2008, Mike Kopetski (D-Salem) who retired in 1994, and Denny Smith (R-Salem) who lost in 1990 to Kopetski because of his ties to failed savings & loans and then went on to lose a race for governor to John Kitzhaber in 1994.
All three counties are represented by Democrats in congress, with Marion and Polk in the 5th district held by Kurt Schrader (D-Canby), and Yamhill in the 1st district held by Suzanne Bonamici (D-Beaverton). All three voted for their Democratic incumbents last fall while also giving a majority of their votes to Mitt Romney for president. Most of the state legislators in the region are Republicans, with the only Democrats being Peter Courtney, and State Reps Brian Clem (D-Salem) and Betty Komp (D-Woodburn). Komp represents the only Hispanic-majority district in the state (in fact the only district without a clear non-Hispanic white majority), but is white herself.
The rest is dominated by Republicans. West and southern Salem are represented by moderate Republicans Senator Jackie Winters and Rep Vicki Berger. The rest of the region varies between traditional conservatives Rep Vic Gilliam (R-Silverton), and Senators Brian Boquist (R-Dallas) and Larry George (R-Sherwood), but also arch conservatives like Senator Fred Girod (R-Stayton), and Reps Jim Wiedner (R-McMinnville), and Kim Thatcher (R-Keizer). I and others have called this the Bible Belt of the Willamette Valley for good reason.
Lets consider the relative size of these areas.
Marion County (2010 numbers): 315,335, or >8% of Oregon's population.
Salem: 154,637 (also in Polk County)
Mt. Angel: 3,286
Gates: 471 (Part in Linn County)
St. Paul: 421
Scotts Mills: 357
Idanha: 134 (Part in Linn County)
Polk County: 75,403, or <2% of Oregon's population.
*Willamina: 2,025 (also in Yamhill County)
*Falls City: 947
Yamhill County: 99,193, or <3% of Oregon's population.
*these cities do not have their own precincts and so there is no political data below.
Marion County has several regions, but most people live in Salem-Keizer. Above that there is northern Marion, with St. Paul, Woodburn, Hubbard, Aurora, Gervais and Donald, which is largely small towns and farms. This area is heavily Hispanic, principally in Woodburn, Gervais, and Hubbard. A bit east of there is Mt. Angel, home of the abbey, and then the farms begin to fade into the foothills of the Cascades. In mountainous eastern Marion county there is Silverton, which is more and more of a tourist destination with the Oregon Garden and Silver Falls State Park, and is notable for having America's first transgendered mayor, once a friend of my father (I grew up just a few miles from Marion County). Other cities in the east include Detroit, Idanha, and Gates heading into the Santiam Pass. East and south of Salem is again more agricultural and rural, with cities like Aumsville, Stayton, Sublimity, and Jefferson anchoring the population. About 215,000 of Marion's 315,000 live in Salem, Keizer, and Woodburn proper, and an additional 35-40,000 in unincorporated suburban-style pockets in eastern Salem. The remainder lives in the cities of less than 10,000 or rural areas.
This region may be the most Hispanic of any in western Oregon, if not all of Oregon, because of substantial populations in Marion County, and some smaller ones in pockets of the other two. Woodburn, Salem, Gervais, and Hubbard in Marion County are very Hispanic, probably because of traditional communities there for migrant farm workers which have grown over the years, and are beginning to have a bigger role in how elections turn out. Still they lag in turnout, for while Marion County has been growing in recent years, most of that growth has come from the Hispanic population, and Marion County's share of the statewide vote has actually declined as a result.
Across the street from the state capitol in Salem is Willamette University, which I believe is Oregon's oldest college, though private. Salem is also home to the religious Corban College, and Chemeketa Community College. Marion County is one of the places in Oregon where agriculture is biggest, but the state is the biggest employer.
West Salem, the portion at the east end of Polk County, was incorporated into Salem in 1949. It's whiter than the Marion portion of Salem, and Republican-leaning, whereas the rest of Salem is now Democratic-leaning. The eastern side of Polk is more urban, with West Salem, but also to the south Independence and Monmouth, the home of Western Oregon University. The central part of the county is more rural and agricultural, with the city of Dallas (the county seat) anchoring it, and agriculture, particularly viticulture, is a big deal there. The western end of the county is even more rural, but mountainous. Falls City and other formerly bustling logging communities reside there in the coast range, some now abandoned.
About half or more of the population lives on the east end, with perhaps 20,000 or so in West Salem, and an additional 18,000 or so in Monmouth and Independence. While West Salem is Republican leaning, the latter two are more Democratic leaning, and the numbers below indicate that they're becoming more so, likely because of Independence's higher than normal Hispanic population and the presence of WOU in Monmouth. Dallas and the central rural areas are more Republican leaning, but not terribly so. Together Dallas, West Salem, and Monmouth & Independence make up about 70% of the county's population. The sparsely populated western end of Polk County is slightly Democratic leaning, like many of the mountainous, historically forestry-driven areas in Oregon are ancestrally Democratic. What may tip the balance here, though, is the Native American population. While not overwhelming, there is enough population from the Grande Ronde reservation and other communities to make the difference. Unfortunately the reservation, like many in Oregon, does not have its own precinct(s), so I don't have data on it alone, or I would present it here.
Both McMinnville, the county seat, and Newberg are small college towns, but are very different. McMinnville is home to Linnfield College, a small, private liberal arts school, and McMinnville is a swingy urb as a result. Newberg, though, is home to the religious George Fox University, has seen a lot of recent growth as its at the outer boundary of the Portland metro area, and from what I can tell is the most conservative city of at least 20,000 people in the state outside of Southern and Eastern Oregon.
Yamhill County is the epicenter of Oregon's wine country. Around both major cities and throughout most of the county there are dozens of wineries and vineyards. The cities in the northeast, Newberg and its satellite Dundee, are becoming more integrated into the metro area, as Newberg has become an exurban destination for white flight from Portland and the inner suburbs as they become more liberal and less white. McMinnville and the rest of the county are really too far from Portland for that, though there are some commuters to the southern and western suburbs. Towards the west end of the county the population is sparse and mountainous, as it extends into the Coast Range. The southwest is different from the rest of the county with Sheridan and its federal prison and the Grand Ronde area's concentration of Native Americans and a casino.
Like the other two counties here, Yamhill leans Republican and is somewhat conservative. McMinnville is one bright spot in the county, as its the only city of the three I looked at which leaned Democratic. The rural areas and smaller cities seem uniformly more Republican leaning, with Newberg being the only large population center which is stubbornly very Republican, and seemingly standing in the way of the county going Democratic more often.
Now, lets consider the relative leanings of the cities in this region. For presidential election years I'm using how that city performed relative to the national popular vote, and for midterm election years relative to the statewide vote for governor. Because of the large number of cities in Marion County, I've split it up into those two categories. Because 1998 featured a landslide election for John Kitzhaber in which he performed particularly well in the Salem area, probably because of state employees, it seems not particularly useful for interpreting the data.
In Marion County there are a few things that one can see from the data. It appears that whiter, smaller communities like Aurora and Detroit (about 11% and 3% Hispanic in 2010) and small agricultural communities have trended more Republican over the years, while heavily Hispanic places like Gervais and Woodburn have not, and in fact Gervais appears to have trended Democratic, while Woodburn has been fairly consistent, and Salem has clearly trended Democratic (67%, 59%, and 20% Hispanic in 2010). Hubbard, though, has not, though Obama won it narrowly in 2008 (36% Hispanic in 2010). Keizer has been oddly consistent. Silverton appears to be trending Democratic, despite being small and white, but probably because of the local tourism economy.
In Polk and Yamhill counties there is less data, but while Newberg, Dallas and West Salem seem to perform rather consistently, Monmouth and Independence seem to have a Democratic trend, and Dundee may as well. For Dundee this may be from focus in the city on wine and tourism.
Across the region it is common to see Democrats performing worse, even relative to statewide and nationally, in midterms compared to presidential elections, particularly more recently. I would think this is because of two major populations: students and Hispanics. All three counties have colleges which generally are more progressive areas (aside from Newberg), and all three have not insignificant Hispanic populations, and Democrats have had a hard time turning both populations out, especially in midterm elections. I think the presence of a large population of public employees prevents a total collapse of Democratic turnout in midterms in the Salem area, as those union members tend to have high turnout.