Second-tier is right, but every election cycle features second-tier races producing upsets.
Ummm, not at a statewide level. I wouldn't say there was any statewide second-tier upsets in 2004. There were two house races, (The Indiana one, Hill? I think that's name, and Melissa Bean's victory), but I no Governors or Senators were randomly upset.
And I guess you could make the case for Sunny Perdue or Chambliss in 2002, but they were top contenders, just against opponents that were considered untouchable.