Could I suggest not voting at all rather than vote for their opponent? I think the total number of votes cast going down would send a more poignant message.
They get a message, but they quite reliably learn the wrong lesson: They think the voters aren't turning out because the candidates aren't "centrist" (read that: "liberal") enough. Voters are "turned off" by "conservative" candidates. This is how we got Romney and McQueeg.