Political actors play an increasingly important role in the dissemination of political information on social media. However, relatively little is known about the mechanisms why specific news items are shared with the support base instead of others. For a timespan between December 2017 and the end of 2018, we combine the analysis of Facebook content from 1,022 politicians associated with 20 political parties from Germany, Spain, and the UK, with an automated content analysis of media coverage from 22 major online news outlets, and survey data in a multilevel binomial regression approach. By comparing news items that have been shared by one or several political parties with news items that have not been shared by any of them, we overcome the selection biases of previous studies in the field of news dissemination. Findings show that a news item's likelihood to be shared by a politician increases (1) if that politician's party is mentioned in the news item, (2) the more salient their party's owned issues are in the news item, and (3) the more party supporters tend to read the news outlet in which the news item is published. We contextualize these findings in light of political actors’ multi-faceted motivations for news sharing on social media and discuss how this process potentially reinforces an information bias that may contribute to the polarization and fragmentation of audiences.

政治行为者在社交媒体上传播政治信息方面发挥着越来越重要的作用。然而，对于特定新闻项目与支持基地而不是其他人共享的机制知之甚少。在 2017 年 12 月至 2018 年底期间，我们将来自德国、西班牙和英国的 20 个政党的 1,022 位政治人物的 Facebook 内容分析与来自 22 家主要在线新闻媒体的媒体报道的自动内容分析相结合，以及多级二项式回归方法中的调查数据。通过将一个或多个政党分享过的新闻项目与没有任何一个政党分享过的新闻项目进行比较，我们克服了以往新闻传播领域研究的选择偏差。调查结果表明，一条新闻“ 被政治家分享的可能性增加（1）如果该政治家所在的政党在新闻项目中被提及，（2）他们政党拥有的问题在新闻项目中越突出，以及（3）政党支持者倾向于阅读的越多发布新闻的新闻媒体。我们根据政治行为者在社交媒体上分享新闻的多方面动机对这些发现进行背景化，并讨论这一过程如何潜在地加强可能导致受众两极分化和分散的信息偏见。