In February 2013, Mehlman helped organize an amicus curiae brief to the Supreme Court in the case of California Proposition 8 signed by dozens of prominent socially moderate Republicans, including Jon Huntsman, Jr., Meg Whitman, Ken Duberstein, Ileana Ros-Lehtinen, and others, explaining that they supported a Constitutional right to same-sex marriage. According to the "New York Times", Ken Mehlman spent "months in quiet conversations with fellow Republicans to gather signatures for the brief." "We are trying to say to the court that we are judicial and political conservatives, and it is consistent with our values and philosophy for you to overturn Proposition 8," said Ken Mehlman according to the New York Times report.