Perhaps the same polarization that has driven Obama's lead to single digits here has also aided Merkley in keeping it close.
Without seeing a regional breakdown its hard to say for sure where the drop for Smith is located (but even then the MOE is too high) but Smith has been successful in the past with his ability to do well (for a Republican) in the Portland metro. Your dealing with what is generally a very well educated area, so they might be more likely to be turned off by the negative ads Leif was mentioning, and just a hardening in support for Merkley as a result og that factor and the increased polarization.
