How Union Sponsorship Raised British Candidates' Vote Shares by Six Points

🗂️ A new roll of every union‑sponsored candidate

This study evaluates how union sponsorship affected the electoral prospects of British parliamentary candidates throughout the 20th century. A new dataset was compiled covering the universe of union‑sponsored candidates to enable this long‑run analysis.

🔎 Research design that isolates sponsorship effects

A difference‑in‑differences design exploits within‑candidate variation created by the sponsorship institution and its later abolition. This identification strategy isolates the causal effect of sponsorship on electoral outcomes.

• Design: within‑candidate difference‑in‑differences based on sponsorship institution and abolishment
• Data: newly assembled universe of union‑sponsored parliamentary candidates

📊 Key finding: a clear boost in vote share

Sponsorship caused an average six percentage point increase in candidate vote shares.

💡 Which mechanisms explain the boost?

Five candidate‑level channels were examined: constituencies, opponents, resources, mobilization, and information. The evidence supports two mechanisms in particular:

• Constituency placement: Sponsorship increased the likelihood that sponsees were nominated in more attractive constituencies, accounting for roughly two‑thirds of the overall effect.
• Resources: Sponsorship generated an inflow of resources into constituency‑party organizations that further raised vote shares.

Evidence was weaker for opponent selection, mobilization, and information channels.

📈 What this means for representation and party power

Sponsorship promoted the election of union‑friendly candidates to Parliament, thereby shaping representation. However, the net impact on the party system was modest: these shifts produced only moderate changes in the balance of power between parties.