A little while back I got into a discussion down south on whether Major League Baseball should be allowed to come into the Portland market. My argument is two-fold. No, because it would split the Mariners' tv market and create two bad teams rather than one OK one. No, because no one it talking about expanding MLB to Mexico or Latin America, which is the next logical move for the sport.
Here are a couple of maps from the CommonCensus Map project that illustrate my first point.
This version of CommonCensus tracks your zip code and then your preference for sports teams. What we see with the baseball map is preference for the Mariners in the Puget Sound region, Yakima and the Tri-cities, the Inland Empire and Portland/Willamette Valley.
For the Sonics, that preference is cut-off at about the Clark County line, and the metro Portland area and the rest of Oregon prefer the Trailblazers.
In addition to that obvious observation, you also see smaller basketball blobs in eastern Washington than you see baseball blobs. The existence of a competitive team in a region seems to drives down the preference for either team. That said, preference for the Blazers seems more widespread and uniform in Oregon than support for the Mariners.
So, I'm still going to go with the "a baseball team in Oregon would hurt the Mariners" argument. Totally selfish, but I'm convinced that Portland is well served rooting for a team in Seattle.