"Senate Democrats are fu$%ing useless". "I am done with them."
Sure, we can try to defeat them in the primaries. We can also try to swim the Atlantic ocean to visit Ireland. What happens when you fail? In SD, for example, Tim Johnson will still be just as useless. By the OP, since I am done with him/them, I cannot hold my nose and vote for him again. So either I vote for Joel Dykstra (R) or I don't vote, or I do a write in, or maybe there's a third party on the ballot (Reform or Libertarian). Logic and math dictate that either Johnson or Dykstra will win - whoever gets the most votes. It is an inescapable fact. If I do not vote for Johnson, then I am helping Dykstra to win. Or look at 2010. Stephanie Herseth Sandlin, who has been called a 'Bush dog', votes with the Democratic party 91.8% of the time. John Thune, OTOH, votes with the Republican party 91.1% of the time. Say Republicans and Democrats vote the same 30% of the time. That still gives me a choice between somebody who votes the way I want 61% of the time and somebody who does it 8.9% of the time. That shouldn't be a tough choice.
http://projects.washingtonpost.com/congress/110/states/sd/