While Oregon was Republican, it wasn't overwhelmingly so. Recall that OR senator Joseph Lane was Breckenridge's running mate in 1860, through Lincoln did barely carry the state with 36% of the vote. Except for 1904, when Roosevelt got 67% of the vote, Republican margins in Presidential elections were normally just over 50% (though they did carry the state 10 straight times from 1872 to 1908).
http://members.cox.net/rbt48/weather/Presidential_Elections/Oregon_pv.pdf But during the Civil War years, it was fairly evenly split, having been represented by Democrats in the Senate for most of the conflict.
http://en.wikipedia.org/wiki/List_of_United_States_Senators_from_OregonAll that being said, Seymour's margin in 1868 was only 164 votes out of 22,086 cast, or a margin of 0.64%. It's perhaps worth noting that Grant carried California by just 0.48%, so it seems his strength in the far West was not overwhelming. As for Seymour's relatively good showings out West, I cannot explain them beyond what I have already presented here.
