One of the more curious things about my search was the dearth of records before the mid-1970's, a couple records from Washington (including specimen records) and nothing official from Oregon. Roy's note on the treatment of kingbirds by the ruling members of the birding community of that period may be the explanation.
I did a bit of rooting around in my own personal records and discovered I have seen 39 Tropical Kingbirds from 24 unique by year and location records on the Lower Columbia (both sides of the river) since 2000. 11 of 39 were multiple birds, as many a 3 interacting together. These were all birds I had reported to either birdnotes, OBOL or ebird. There are probably a handful more I could find if I took the time to dig through old notebooks or could search OBOL records from before 2000.
By contrast I have only seen Western Kingbird in the Spring and Summer. And I've seen more Tropical Kingbirds on the Lower Columbia than Eastern Kingbirds, Say's Phoebes, or Ash-throated Flycatchers.