Why do trade unions organize antigovernment strikes in some countries but not in others? This article argues that there is a curvilinear relationship between union density and political strike activity. Political strikes are rare in countries with low union density, since effective protests require a basic level of organizational capacity. They are also rare in countries with high union density, since a government that faces a strong union movement has powerful incentives to adjust its policies in order to avoid open confrontation. But political strikes are relatively common in countries with moderate levels of union density, since it is difficult for governments and unions to find viable compromises when the strength of the unions is not secure. The empirical part of the article estimates the relationship between union density and the likelihood of political strikes in two samples of advanced democracies.
That is the abstract of a paper by Johannes Lindvall, I did not find the full paper online.