I missed this Pew survey when it was first released in October 2006. But its findings are quite intriguing. Among self-identified Democrats, as recently as 2002 as many Democrats considered themselves conservatives as liberals. Since then there has been a significant increase in the share of liberals. While overall ideological trends appear stable, as I noted in an earlier post, there is other research that reports a strong increase in support for the Democrats.
A key ambiguity in the survey is that is uses the designation "moderate" to capture those in the middle. One problem with this is that the word "liberal" has been demonized for a number of years in popular and media discourse. My guess is that if one parsed the moderates, one would find that, in substance, more are really liberal than conservative, both among Democrats and among the population as a whole. This would account, for example, why overall there has been an even partisan division within the U.S. for the past number of years despite a nominal advantage for conservatives. The trend at the moment is to a Democratic advantage.