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Online social networks constitute a major platform for the exchange of moral and political ideas, and political elites increasingly rely on social media platforms to communicate directly with the public. However, little is known about the processes that render some political elites more influential than others when it comes to online communication. Here, we gauge influence of political elites on social media by examining how message factors (characteristics of the communication) interact with source factors (characteristics of elites) to impact the diffusion of elites’ messages through Twitter. We analyzed messages (N = 286,255) sent from federal politicians (Presidential candidates, Senate, House of Representatives) in the year leading up to the 2016 U.S. presidential election – a period in which Democrats and Republicans sought to maximize their influence over potential voters. Across all types of elites, we found a ‘moral contagion’ effect: elites’ use of moral-emotional language was robustly associated with increases in message diffusion. We also discovered an ideological asymmetry: conservative elites gained greater diffusion when using moral-emotional language compared to liberal elites, even when accounting for extremity of ideology and other source cues. Specific moral emotion expressions related to moral outrage—namely, moral anger and disgust—were particularly impactful for elites across the political spectrum, whereas moral emotion expression related to religion and patriotism were more impactful for conservative elites. These findings help inform the scientific understanding of political propaganda in the digital age, and also the potential antecedents of political polarization that is rapidly growing in American politics.