It may or may not be true that both those who ordinarily would vote Democrat and those who ordinarily would vote Republican are equally likely to vote Libertarian instead, all other things being equal; I don't know of any survey that has tried to find this out, though it's likely one exists. The Libertarian Party is certainly a good opportunity for a protest vote against the drug policy (among other policies) favored by both Republicans and Democrats, but my impression is that the Libertarians' biggest draw is their proposed lower taxes, which is also a major concern for most Republican voters.
Whether or not my impression is correct, the main thing I see skewing the results is the presence of the Green Party, which is a high-profile alternative for those voters who are socially more liberal than the Democrats but fiscally less conservative than the Libertarians. There are many small parties that cater to both the right and left, but by far the two that get the most votes are the centrist Libertarians and the liberal Greens (I voted a nearly straight Natural Law ticket in 1996 and 1998, but that was only because the Green Party wasn't on Michigan's ballot at the time). Because the Greens draw so many socially liberal votes from the Libertarians and because, especially with the lackluster Buchanan Reform Party bid, there isn't a large conservative Green equivalent, it seems more likely that a relatively big Libertarian win will affect the Republican Party more than it does the Democrats. I would even go so far as to predict that the Libertarians will only get a large precentage of the vote with a large Republican defection.
Rebuttals are welcome. Naturally, I would prefer it if Republicans believed your essay and not mine, since yours does a better job rationalizing their defection from their party of choice. Ah, well.
—Rip Van Wormer