Oregon has five seats in the House, apportioned according to the 2000 United States Census. Its 2007–2008 congressional delegation consisted of four Democrats and one Republican. This remains unchanged although CQ Politics had forecasted district 5 to be at some risk for the incumbent party earlier in the year.
A primary election for Democrats and Republicans was held on May 20. To be eligible for the primaries, candidates had to file for election by March 11. Other parties had other procedures for nominating candidates.
In the Republican primary, Joel Haugen defeated pathologist Claude W. Chappell IV, but later withdrew his acceptance of the Republican nomination after his endorsement of Democrat Barack Obama for President drew objections from Republican party leaders.
Incumbent Democrat Earl Blumenauer has represented Oregon's 3rd congressional district since 1996 and was the Democratic nominee in 2008, defeating TV co-host John Sweeney and retired utility worker and peace activist Joseph "Lone Vet" Walsh in the primary. In the general election, he faced Republican Delia Lopez, a real estate investor, and Pacific Green Party candidate Michael Meo.
Incumbent Democrat Peter DeFazio has represented Oregon's 4th congressional district since 1986 and was unopposed for the Democratic nomination in 2008. He was being challenged in the general election by Constitution Party member Jaynee Germond and Pacific Green Mike Beilstein, a research chemist. CQ Politics forecasted the race as 'Safe Democrat'.
In February 2008, Democrat Darlene Hooley, who had represented Oregon's 5th congressional district since 1996, announced that she would not seek re-election in 2008. The race to replace her was expected to be one of the most competitive in the nation, since the district contained about 2,000 more Republicans than Democrats at that time.
There were two major factors for the competitiveness of the race: first, the demographics of the district had changed dramatically. In June, there were 20,000 more registered Democrats than Republicans in the district, a net swing of 22,000 voters since February. Secondly, Republican nominee Erickson won a contentious primary in which an opponent, Kevin Mannix, raised an allegation that Erickson paid for a former girlfriend's abortion. The girlfriend subsequently went public with the information, but Erickson denied knowledge of the event. Mannix refused to endorse Erickson in the general election.