Saumya Prabhat and David Primo have posted this draft (a version of which will be presented at the APSA Law and Political Process Study Group panel on campaign disclosure).  Here is the abstract:

We utilize a quasi-natural experiment to examine whether disclosure and shareholder approval of political expenditures reduces shareholder risk. In particular, we examine the Neill Committee Report (NCR), which led to the passage of the United Kingdom’s Political Parties, Elections and Referendums Act 2000 (PPERA) and strengthened disclosure of and required shareholder approval for campaign contributions. Using a differences-in-differences methodology, we find that politically active firms saw an increase in their stock’s volatility along with negative long-term abnormal stock returns upon the release of the NCR. These results present a challenge to arguments for greater shareholder oversight of corporate political activities.