COVID-19 has reinvigorated the policy debate for a universal healthcare system, attracting much attention on social media. In this paper, we study the online discourse of Medicare-For-All before and after COVID-19 by examining the Twitter feeds of two opposing health advocacy groups-Physicians for a National Health Program (PNHP) and Partnership for America's Healthcare Future (P4AHCF). Our empirical results show a sharp contrast between the two interest groups' communication strategies. PNHP showed a consistent narrative before and after the onset of COVID-19 on 11 March 2020, marked by personalized stories, references to diverse demographic groups, and a growing number of Medicare-For-All tweets. In contrast, P4AHCF showed more scientific terminology and data-centric tweets and had an inconsistent narrative with a sudden surge in positive sentiments and a complete silence on Medicare-For-All right after 11 March. The difference in communication strategies is consequential. PNHP has higher engagement with Twitter users and is more adaptive to a pandemic narrative than P4AHCF. We discuss how distinctive social media strategies can be explained by the groups' different audiences and resources. The findings add to our understanding of healthcare advocacy campaigns on social media and the implication of a pandemic for health policy reform.