Moral foundations theory (MFT) has been a useful framework for understanding moral judgment and its relationship to political leaning. However, some have argued that MFT omits key domains of moral reasoning. We explored the utility of two candidate foundations (Proportionality and Equity) with a national sample of U.S. adults recruited through Nielsen’s Harris Panel, randomly split into calibration ( n = 1,499) and replication samples ( n = 1,499). We find that Proportionality and Equity are conceptually distinct from the original foundations (as measured in the Moral Foundations Questionnaire [MFQ]) but relate to them in predictable ways. Equity consistently predicted political leaning above and beyond covariates and the original foundations, but Proportionality only distinguished conservatives from liberals in the calibration sample, which suggests Proportionality may be highly relevant to moral judgments regardless of political ideology. Our findings also indicate potential bias when using one of the MFQ’s screener items to filter out unengaged participants.