In early 2019, a US climate change and economic renewal policy proposal called the Green New Deal (GND)1 rose from obscurity to national prominence in just four months. This situation created a natural field experiment in which to study the emergence of partisan polarization. Here, we report findings from two nationally representative surveys of registered US voters that measured familiarity with and support for the GND shortly before and after the issue entered the national spotlight. Initially, there was low public awareness of the GND but majority support for it across party lines. Four months later, voters had become much more familiar with the GND and partisan polarization had increased significantly due to a sharp decrease in support among Republicans. In fact, Republicans who had heard the most about the GND were the least likely to support it. In contrast, support for the GND remained high among Democrats, and did not vary substantially across degrees of familiarity. We also identify a likely mechanism: a ‘Fox News effect’. That is, among Republicans, Fox News viewing was a significant predictor of both familiarity with the GND and opposition to it, even when controlling for alternative explanations.