Liberals and libertarians finally break up.

One mini-saga of the past decade in American politics has been the flirtation—with talk of a deeper partnership—between progressives and libertarians. These two groups were driven together, in the main, by common hostility to huge chunks of the Bush administration's agenda: endless, pointless wars; assaults on civil liberties; cynical vote-buying with federal dollars; and statist panders to the Christian right.

This cooperation reached its height during the 2006 election, in which, according to a new study by David Kirby and David Boaz, nearly half of libertarian voters supported Democratic congressional candidates—more than doubling the support levels from the previous midterm election in 2002. (As Jonathan Chait noted after the first Kirby/Boaz study of libertarian voting, their definition is overly broad, encompassing 14 percent of the electorate.) At the time, left-wing blogger Markos Moulitsas hailed the influx of "libertarian democrats" into the Democratic coalition. Soon, even the Cato Institute's Brink Lindsey was proposing a permanent alliance of what he called "liberaltarians."

Well, you can say goodbye to all that. The new Kirby/Boaz study reports that libertarian support for Democrats collapsed in 2008, despite many early favorable assessments of Barack Obama by libertarian commentators. Meanwhile, the economic crisis has raised the salience of issues on which libertarians and Dems most disagree. And there's no question that during Obama's first year—with the rise of the Tea Party movement and national debate over bailouts, deficits, and health care—libertarian hostility to the new administration has grown adamant and virtually universal. But what progressives need to understand is that the end of this affair is actually a good thing.

The progressive-libertarian alliance may have provided tactical benefits in 2006, augmenting the Democratic “wave” election of that year. But 2008 showed that libertarian support is hardly crucial: Obama still won "libertarian" states such as Colorado and New Hampshire handily, even without their backing, and he generally performed better in the “libertarian West” than any Democratic nominee since LBJ.

In terms of a deeper bond based on philosophical congruence, it’s true that modern liberals and libertarians share common ideological roots in eighteenth and nineteenth century Anglo-American liberalism. Both believe in a world of rational actors, and both consider the promotion of individual autonomy to be a positive good. With the emergence of the "neo-liberal" and "New Democrat" movements of the 1980s and 1990s—which lauded capitalism, technological progress, and free trade—the potential for overlap only increased.