Obama's "Drop" in Oregon and SUSA sampling.

The latest SurveyUSA poll (conducted June 17th to June 19th of 547 likely voters) of Oregon has Barack Obama narrowly leading John McCain 48-45.

This seems like a significant drop from the poll a month ago (May 16 to May 18th of 600 likely voters) which had Senator Obama leading John McCain 49-39

However, a look at the crosstabs makes it seem like differences in the poll results are due mostly to changes in the samples.

By Party:

Party May June
Dem 48% 42%
Rep 32% 41%
Ind 20% 15%

Such a shift in the Republicans favor is highly unlikely (a shift in the Democrats favor would still be unlikely, if a bit less so).

Which one is right?

Well, currently in Oregon, Democrats have a 43% to 33% registration advantage, so one expects the correct party divisions are somewhere in between the May and June numbers.