Systematic field surveys were conducted in 2010–13 for the threatened, globally rare plant species, Helonias bullata L. (swamp pink: Liliaceae) in New Jersey, which contains 60% of known populations. A total of 90 sites were visited, including 56% of extant populations, focusing on the highest quality, largest, and most geographically representative populations in the state in order to comprehensively assess the species' status. A complete census and threat assessment was conducted for each population, using standardized methods adopted during previous surveys in 1997–2001, and population trends were determined by comparing results between data sets. The results found declines in 83% of Helonias populations in the past 10 yr, including severe declines (> 67%) in 53% of populations, and another 26% experiencing declines of 20% or more. Analyses of past and present data sets found the primary cause of declines appeared to be herbivory by white-tailed deer (Odocoileus virginianus), which had not been previously regarded as a significant threat to the species, followed by hydrological changes caused by human development and/or water consumption. Browse damage to Helonias was directly observed in 84% of populations, with an average of 40% of Helonias clusters exhibiting recent damage at each site. Concern for observer bias was also explicitly addressed in the methods and analyses providing further confidence in the results. Implications for the conservation and recovery of Helonias are discussed, as well as the benefits and limitations of the standardized methods used for long-term studies of this and other rare plant species in the future.