This paper examines the relationship between unofficial party blogs and official party sources in the UK using a mixed-method approach. Specifically we combine interview data with content analysis, user surveys and usage data, and finally hyperlink analysis to profile the emergence, popularity, audience and online prominence of four major party blogs since 2005. The core question posed is how far the blogs are challenging parties as the focal point for member activism and offering an alternative public 'voice'. The findings show blogs occupy an important alternative critical space for party debate, particularly outside elections. They are not mobilizing tools, however, being used by the grassroots largely for information-gathering and discussion purposes.