ISTR that Stephanie Herseth (D) had a sizable lead over Diederich (R) in the polls before the Repubs played the "concession"/"low expectations" card...the race ended up being a squeaker with Herseth barely winning.  I haven't analyzed that election in depth, so maybe I'm missing other factors, but it does seem like a shrewd strategy.  People are distressingly lazy sometimes, and if you convince them their votes are worthless by instilling a sense of complacency, apathy, or demoralization, then some fraction of them will probably stay home.