A longitudinal case study of Korean white-collar labor movements, which newly thrived in the democratizing atmosphere after the 1987 June Democratic Struggle, confirms that political opportunity is an important external factor that impels movement dynamics toward political protest and interunion solidarity. However, the impact of political opportunity is more complicated than the political process model suggests. First, it is not objective but perceived opportunity that is causal for movement dynamics: Opportunity is filtered through participants' interpretations, which shape their responses to it. The effect of political opportunity is mediated by participants' subjective conclusion (often inaccurate) that a movement goal has been promoted or obstructed by a particular source (source attribution). Without this framing mediation, the impact of political opportunity remains indeterminate, as a single opportunity structure may produce disparate movement dynamics and, conversely, movements may mobilize under both contracting and expanding opportunities. Second, the causal impact of perceived opportunity–whether perceived contraction or expansion–is contextually specific and contingent. When union members consider their attempts to achieve goals a failure and ascribe the failure to government intransigence, anti-government sentiments facilitate political protest. In contrast, success attributed to the efficacy of collective action nurtures solidarity consciousness and labor collectivity. In either event, movement dynamics improve.