Recent research has advanced our understanding of mass-level affective polarization, defined in terms of partisans’ identity-based emotional reactions to the two major parties in the U.S. Among the mass public, affective polarization has risen in recent decades and appears to carry real consequences for mass opinions and behavior. Less is understood about whether political elites are similarly affectively polarized and the consequences this carries for elite behavior and representation. In this paper, I propose a novel measure of elite partisan messaging and affective polarization which I call the Index of Messaging and Affective Polarization (IMAP). IMAP captures both the prominence and direction of sentiment toward the two major parties in tweets posted by elected officials. After describing the measure, I demonstrate its face validity using prominent national political figures to show that the measure comports with their reputations as extreme partisans. Next, I illustrate how IMAP relates to a variety of behavioral outcomes, including congressional bill co-sponsorship decisions, appearances in the partisan media, and public responses to partisan scandals. I then show that the validity of IMAP extends to state governors. Finally, I conclude by considering potential avenues for future research employing IMAP.