Since the Supreme Court decided that the Affordable Care Act's (ACA) Medicaid expansion is optional for the states, several have obtained federal approval to use Section 1115 waivers to expand Medicaid while changing its coverage and benefits design. There has long been concern that policy making for Medicaid populations may lack meaningful engagement with low-income constituents, and therefore the ACA established a new process under which the public can submit comments on pending Medicaid waiver applications. We analyzed 291 comment letters submitted to federal regulators pertaining to Medicaid Section 1115 waiver applications in the first five states to seek such waivers: Arkansas, Indiana, Iowa, Michigan, and Pennsylvania. We found that individual citizens, including those who identified as Medicaid-eligible, submitted a sizable majority of the comment letters. Comment letters tended to mention controversial provisions of the waivers and reflected the competing political rhetoric of "personal responsibility" versus "vulnerable populations." Despite the fact that the federal government seemed likely to approve the waiver applications, we found robust public engagement, reflecting the salience of the issue of Medicaid expansion under the ACA. Our findings are consistent with the argument that Medicaid is a program of growing centrality in US health politics.