Many hoped that social networks would allow for the open exchange of information and a revival of the public sphere. Unfortunately, conversations on social media are often toxic and not conducive to healthy political discussion. Twitter, the most widely used social network for political discussions, doubled the limit of characters in a Tweet in November 2017, which provided a natural experiment to study the causal effect of technological affordances on political discussions with a discontinuous time series design. Using supervised and unsupervised natural language processing methods, we analyze 358,242 Tweet replies to U.S. politicians from January 2017 to March 2018. We show that the doubling the permissible length of a Tweet led to more polite, less informal, more analytical, and overall healthier discussions online. However, the declining trend in the political relevance of these tweets raises concerns about the implications of the changing norms for the quality of political deliberation.