This paper studies partisan differences in medical practice over 1999-2019. I link physicians in the Medicare claims data with campaign contributions to identify party alignment. In 1999, there are no partisan differences in medical expenditure per patient. By 2019, Republican physicians spend 13% more, or $70 annually per patient. I examine four potential sources of this partisan gap: practice characteristics (i.e., location and specialization), patient composition, preferences for financial gain, and beliefs about appropriate care. Even among physicians in the same location and specialty treating patients for the same condition, Republican physicians spend 6% more, especially on elective procedures. Using a movers design, I also find large partisan differences for treating the same patient. I find no evidence that these differences are driven by profit incentives. Instead, the evidence points to diverging beliefs among newer cohorts of partisan physicians. Recent Republican physicians adhere less closely to clinical guidelines, consistent with their reported beliefs in prior surveys. The timing of the divergence matches the politicization of evidence-based medicine. These results suggest that polarization in health policies has led to corresponding divisions in the beliefs and practice of medical professionals.