Located at the nexus between politics and the digital, this PhD thesis wants to shed light on the changing dynamics, opportunities and challenges citizens and parties are confronted with due to ongoing technological changes. In particular, I am investigating e-expression as a new form of political participation online, party responsiveness to voters’ preferences expressed across social media platforms as well as the dynamics of issue competition on Twitter:  In my first paper “E-participation in a comparative perspective: drivers and constraints of expressing political views online”, I examine the conditions under which citizens express themselves politically on the Internet by focusing on individual-level motivations and resources, but also bringing back contextual factors such as media dependence, democratic longevity, civil society robustness and electoral events. I am employing multilevel modelling of ISSP 2014 data for 34 countries and reveal that citizens – particularly on the ideological extremes and distrusting politicians – who are residing in established democracies with an independent media are more likely to engage in e-expression. Moreover, I show that the predictive effect of political distrust on e-expression ceases in countries with less free and dependent media. In terms of the contextual prerequisites, therefore, e-expression follows a similar pattern as offline political participation. Yet, it attracts critical citizens on the extreme ends of the ideological scale. In times of already rising populism and (cyber)polarization this finding points to risks for the future health of democracy. In the second paper “Trending campaigns? User preferences and parties’ issue emphasis across social media”, I connect the demand and supply side in digital politics and investigate the extent to which citizens expressing themselves on Facebook and Twitter do have an influence over the issue packages that political parties promote on those platforms during the German electoral campaign of 2017. Therefore, I hand-coded all Facebook posts and Tweets that the seven biggest German parties published on their account during the heated days preceding the election and conducted a supervised sentiment analysis of all Tweets commenting on the election and Facebook comments on parties’ posts. A Poisson time series regression was used to analyse the data. Thereby, I take a cross-media approach that is sensitive to the technical opportunity structures and audiences different social media inhabit. My findings reveal that the preferences users express are not considered equally on the two social media platforms and for all issues. This suggests that German parties take their technical peculiarities and audiences into account and act strategically according to the type of issue in focus. It is mainly on Twitter and for welfare and economic issues that parties respond to users. Since Twitter is populated mainly by political opinion leaders and journalists, this raises questions on the extent to which cyber optimistic accounts on the potential of digital tools to close the ranks between parties and voters are and can be realized. The third paper “Short-term issue emphasis on Twitter during the 2017 German election: a comparison of the economic left-right and socio-cultural dimensions” is a collaboration with Andrea Ceron and Luigi Curini from the University of Milan. This paper underlines the importance of taking a time-sensitive approach when investigating issue competition. Based on the German election campaign of 2017 and the above-mentioned data on parties’ issue emphasis on Twitter, we reveal that the attention parties give to issues they are renowned for fluctuates frequently and alternates with other thematic priorities that may not be owned. This contradicts earlier accounts of the issue ownership theory. We argue that the dynamics are driven by exogenous shocks and spatial considerations. The exact mechanism behind parties’ decision to emphasize a specific issue in the short run depends on the type of issue. While exogenous shocks and the behaviour of ideologically adjacent rivals incentivizes parties to increasingly tweet about traditional economic and welfare issues, addressing green libertarian and immigration-related socio-cultural issues depends on focusing events. Uncontested socio-cultural issues strongly advocated but mainly owned by a single party remain unaddressed by its rivals unless they enjoy high public salience. The opposite is true for EU-related topics. As a cumulative dissertation, each of my three papers examines digital politics from a different disciplinary angle, analytical level and employing diverse methodological approaches: while the first paper is located in the field of comparative politics and political behaviour using survey data, the second and third paper are focusing on political communication and party politics based on large-N Big Data of the digital traces left by political parties and citizens on social media platforms. Altogether, therefore, my dissertation aims at shedding light on the complex transformations of politics in the digital era on the micro, meso and macro-levels as well as their interactions, which has important implications for the study of representative democracy. Overall, my findings suggest that politics is affected by digital communication, but not fundamentally reshaped.