The use of negative political communication is a predominant characteristic of modern politics. However, literature doesn’t provide an answer to the following question: what explains fluctuations in the use of negative messages within political organisations during a given political campaign? The present paper examines this question in the context of the 2014 Scottish independence referendum. Data consists of all tweets distributed by the official Twitter account of both campaign organisations (@YesScotland and @UK_Together) between June 16, 2014 and September 17, 2014. Results are obtained by a non-parametric local regression and by time-series regression analyses. Our model demonstrates that having an advance in the polls had a statistically significant influence on the tweet sentiment of at least one organisation during the referendum campaign: Better Together’s messages were more negative when it was ahead in the polls. Meanwhile, Yes Scotland’s messages were more negative after each of the leaders’ debates.