In past election cycles, I really haven't had a strong preference among the candidates. I voted for the Bernstein/Bernstein ticket in 2004, and can't really remember who I voted for, or for that matter whether I voted, from 1988 to 2000. But I'm much more of a Republican partisan this time, for a few reasons:
(1) Libertarians have been heavily involved in some of the most important constitutional Supreme Court litigation of the last two decades, either in terms of bringing the case, being among the most important advocates of one side's constitutional theory, or both. Among the cases in this category are Lopez, Morrison, Boy Scouts v. Dale, U.S. Term Limits, Grutter, Gratz, Kelo, Raich, Heller, and probably a few more that I'm not thinking of offhand. With the minor exception of Justice Breyers' vote in Gratz, in each of these cases, the ONLY votes the libertarian side received were from Republican appointees, and all of the Democrat appointees, plus the more liberal Republican appointees, ALWAYS voted against the libertarian side. The latter did so even in cases in which their political preferences were either irrelevant (Term Limits), or should have led them to sympathize with the plaintiff (Lopez, Kelo, Raich).
The only exception to this pattern is Lawrence v. Texas, in which Justice Kennedy seems to have been influenced by the Cato Institute's brief. But if the liberals had been able to muster five votes without Kennedy, I'm sure the opinion would have been quite different, less libertarian and more about "tiers of scrutiny" and whatnot. I'm a law professor, teach constitutional law, and the subject is dear to my heart. I'd much rather have the side that tends to take my ideological compatriots' constitutional arguments seriously on the Court. And Raich and Kelo, respectively, suggest that the liberals on the Court not only don't take libertarian arguments seriously, they don't believe in (a) any limits in federal regulatory power, whatsoever; or in (b) property rights, even when big corporations are using the political process to screw over the little guy.
(2) I'm not exactly a huge McCain fan. Indeed, other than Huckabee, he was probably my least-favorite choice in the Republican field. But as things have turned out, and despite some absurd, statist, campaign planks, in the home stretch he's running the most rhetorically libertarian presidential campaign I can remember since Reagan's 1980 campaign. Every time I hear a clip on the news, he's denouncing Obama for being a big spender and a taxer. He pledges to freeze most federal spending, and to take on entitlements and the grotesque reverse Robin Hood farm programs that Obama and almost all Congressional Democrats support. If he pulls out a victory, it will be seen as a stunning come from behind victory for those ideas. If he loses, and especially if loses badly, it will look like Americans are okay with "spreading the wealth."
(3) I think there are two great moral issues in American politics today, the disastrous War on Drugs, and free trade. The War on Drugs, for now, is hopeless. Free trade though, is not. Over the past couple of decades, a (statistical) billion people, more or less, have moved from poverty to the local middle-class because of globalization and free trade, far more people than have been aided by all the liberal do-goodism Obama, or any else, has or can muster. McCain is the candidate of free trade; Obama is the candidate of "fair trade," which in practice means protectionism. McCain's policies have the potential to rescue tens of millions of additional people from poverty, who will stay mired there under Obama. (And I always had at least one soft spot for Bill Clinton, for standing up to the unions and the know nothing wing of his party in favor of free trade and NAFTA).
(4) Nancy Pelosi. Harry Reid. Filibuster-proof majority. 'nuff said.
(5) I really still don't have any idea of what Obama is about. Is he the moderate, utilitarian, empiricist that Cass Sunstein raves about? Or is really quite far to the left ideologically, as various aspects of his biography suggest, but just skilled at hiding it for electoral advantage, with his very steady, moderate personality serving as a mask? The last thing the U.S. needs is a left-wing Ronald Reagan, but that might well be what we get.