This new poll, in the field Oct 15-16, shows Braley taking a slim lead:
http://www.lcv.org/...
One dynamic that may be helping Braley is that he's holding his own among age 65+ voters, trailing by only two percentage points. Male and female voters don't seem to differ much, though, which seems odd. On the whole, the linked "internals" don't go into as much detail as usual.