"…None of these involved the typical 1 or maybe 2 percent you ordinarily expect a Lib to garner: Looking at the three-way vote, all but one were over 3 percent, and three took 6 percent or more, with a high of 6.5 percent in the Montana Senate race. These definitely seem like unusually high figures.
"So what’s going on here? …a growing proportion of conservative-leaning voters too disgusted with the GOP to pull the Republican lever, but who won’t vote for Democrats either, are choosing a third option and going Libertarian instead."
http://www.dailykos.com/story/2012/11/15/1162077/-Libertarians-provided-the-margin-for-Democrats-in-at-least-nine-elections#