Last week I looked at a Pew Research Center analysis of libertarians and other ideological groups in the United States. Using the Pew results, let’s compare libertarians to liberals as way of speculating about the upcoming election.

The two groups have a lot in common demographically. Both libertarians and liberals are more likely than other groups to be found in the highest income groups and to have a college education (though liberals are by far the most educated group). Strikingly, both groups are the least likely among the Pew respondents to be supported by African-Americans.

Libertarians and liberals are the least religiously observant among the Pew groups. Both are much more likely than others to say they are “secular” and to state that they “seldom or never” attend church. This secular tendency showed up when Pew looked at responses by groups to some questions about issues. Libertarians and liberals were about as likely to believe homosexuality should be accepted, to favor stem cell research, and to oppose more restrictions on abortion. (Liberals more so on each issue). In each case, the two groups oppose positions endorsed by Roman Catholicism or evangelical Protestant groups. Judged only by these issues, libertarians and liberals should be quite similar politically.

But they are not, and you can guess why. The two differ over the scope of government power over economic life. The Pew data defines liberals in part by their faith that government regulation is needed to protect the public interest. Liberals also oppose tax cuts much more than any other group and to think businesses make too much profit. Liberals are about as likely as any American to believe free trade is good for the nation, their only significant departure from what you might expect on the economic front.

This difference on economic policies translates into partisan differences. Exactly half of libertarians in the Pew survey identified themselves as Republicans or leaning that way. 57 percent voted for President Bush in 2004. When forced to choose, these voters seem to assign more weight to the economic aspects of liberty than the social side.

On the other hand, 41 percent of the Pew libertarians identify with, or lean toward, the Democratic party. Could that number rise?

Here’s one guess, popular with libertarians angry with the GOP. Libertarians are close to liberals demographically and on social issues, and liberals are overwhelmingly Democratic. If Republicans support economic policies similar to liberal positions, libertarians have little reason to vote Republican. The only difference between the parties would concern social questions, and on those issues, the Democrats are more often if not always more libertarian than Republicans.

Here’s the problem with this guess about the future. Libertarians have to consider what Republicans and Democrats will do in the future, and rationally, they need not simply extrapolate from the recent past to do that.

It is true that Republicans have expanded government rapidly to maintain control of Congress (among other reasons). But that does not mean Democrats in power would not expand government even more rapidly for reasons of ideology and interest. Democrats complained that the Medicare prescription drug benefit was not generous enough, not that it expanded government too much. More generally, the American National Election Survey found in 2004 that two-thirds of self-identified liberals and 56 percent of Democrats believed “it is important for the government to provide many more services even if it means an increase in spending.” This support for expanding government, by the way, is the highest ever reported for both liberals and Democrats. It is almost 40 percent higher for liberals and 47 percent more for Democrats than it was in 1996 when Bill Clinton declared the end of big government. If Democrats in power listen to their base in 2007 (or 2009 with the presidency in hand), they will not be responding to the moderates at the Democratic Leadership Council.

Given all that, I don’t think many libertarians who have been voting Republican are likely to start voting Democratic. As bad as the Republicans have been on limiting government, libertarians primarily concerned with economic liberty have reason to think the Democrats would be worse. So they will continue doing what they have been doing unless the Democrats start looking better or the Republicans become even worse.