Historically, Merkley's narrow primary win is good news for Smith

Jeff Merkley may have won a 45%-41% victory in yesterday’s Democratic primary, but it’s rare for a challenger who barely ekes out a primary win to oust incumbent United States Senators in the general election.  Of the twenty challengers to unseat incumbents in U.S. Senate races since 1996, only one did so after winning a primary by a narrower margin than Merkley did.  That was in 1996, when Peter Fitzgerald won a 52%-48% victory over Loleta Didrickson in the Illinois GOP primary, and then went on to defeat incumbent Carol Mosely-Braun. 

In 2006, Jim Webb defeated Harris Miller 53%-47% in the Virginia Democratic primary before unseating incumbent George Allen.  Only two successful challengers won primaries by a lesser percentage than Merkley: Mark Dayton (Minnesota, 2000) defeated Mike Ciresi 41%-22, and Maria Cantwell (Washington, 2000) won the Democratic primary over Deborah Senn by a 37%-13% margin. (In the general election, they defeated Rod Grams and Slade Gorton, respectively.)

In 1998, Charles Schumer (New York) and John Edwards (North Carolina) each won 51% in the Democratic primary (before defeating incumbents Alphonse D’Amato and Lauch Faircloth, respectively), but these were multi-candidate fields. Schumer won 51%-25 over Geraldine Ferraro, and Edwards defeated N.G. Martin by a 51%-28% margin.