The focus of this research is to analyze the activity on Twitter, during the campaign for the 2015 general elections in Spain, of the leaders of the main Spanish political parties: Mariano Rajoy (PP), Pedro Sánchez (PSOE), Pablo Iglesias (Podemos) and Albert Rivera (Ciudadanos). That means: how many messages and what types of messages are published; what topics they focus on; and the main features of using Twitter: to spread political messages, reference candidate activities in the media or on the campaign trail or emphasize personal aspects. The methodological approach establishes three complementary areas: a) a quantitative analysis of the tweets published by the four candidates throughout the election campaign, focused on establishing the percentage of responses and retweets; b) a content analysis to determine the issue agenda of each candidate; and c) a qualitative analysis that allows us to provide an overview of the communication preferences and profile of each candidate. The combined results of this triple methodological approach show that candidates from emerging parties tend to send messages to mobilize their supporters to campaign and to make generic announcements regarding their future victory and the arrival of political change, while the leaders of PP and PSOE tend to publish more messages with specific policy proposals.