It was much more significant in the 2006 elections than the white evangelical vote. In 2006, a full 36 percent of self-described libertarians voted Democrat - easily the biggest share of that vote that the Dems have had in recent times. In general, the libertarian move away from Christianist big government Republicanism has been intense these past six years of Bush. In 2000, Bush won 72 percent of the libetarian vote; in 2004, he won 59 percent. In 2002, the Republican advantage over Democrats among libertarians in the Congressional elections was 47 percent. By 2006, the gap had narrowed to 23 percent. Cato argues that the libertarian vote is about as large as the Christianist vote, and subject to swings three times as powerful. You can read the PDF report on the data here.