Overcoming the COVID-19 pandemic requires motivating the vast majority of Americans to be vaccinated. However, vaccination rates have become politically polarized and a substantial proportion of Republicans have remained vaccine-hesitant for months. Here, we explore how endorsements by party elites affect Republicans’ COVID-19 vaccine intentions and attitudes. In a pre-registered survey experiment (N = 1,480), we varied whether self-identified Republicans saw endorsements of the vaccine from prominent Republicans (including video of a speech by former President Donald Trump), from the Democratic party (including video of a speech by President Joseph Biden), or a neutral control condition including no endorsements. Unvaccinated Republicans who were exposed to the Republican elite endorsement reported 7.0% higher vaccination intentions than those who viewed the Democrat elite endorsement and 5.7% higher than those in the neutral control condition. These effects were statistically mediated by participants’ reports of how much they thought Republican politicians would want them to get vaccinated. We also found evidence of backlash effects against Democratic elites: Republicans who viewed the Democrat elite endorsement reported they would be significantly less likely to encourage others to vaccinate, compared with those who viewed the Republican elite endorsement or the neutral control. These results demonstrate the relative advantage of cues from Republican elites - and the risks of messaging from Democrats currently in power - for promoting vaccination among the largest vaccine-hesitant subgroup in the U.S.