Increasing partisan polarization has characterized American politics for decades. On January 6, 2021, both Republicans and Democrats in Congress expressed their horror at the violent invasion of the US Capitol, leading to the popular perception—emphasized by media accounts—that the attack generated a rare moment of bipartisan unity. We argue that while members of both parties condemned the attack, a marked partisan divide characterized their messaging even as events unfolded. We analyze all 1861 tweets by members of Congress on January 6th and find that Republicans were significantly more likely to characterize the invasion as a protest grown out of hand, while Democrats described it as an attack on democracy. The results strongly indicate that partisan polarization was alive and well on January 6th and may help to account for Republicans’ shift toward normatively positive portrayals of the day in subsequent months.