A group of Christian investors are warning some of the largest U.S. retailers including Costco and Walmart against selling the abortion pill mifepristone.

Companies offering the drug risk reputation and legal repercussions, according to an Aug. 2 letter sent to CEOs at the two retail giants, as well as Kroger, Albertsons and medical distribution company McKesson Corp. The group said 6,000 Costco customers have signed a petition saying they will cancel their membership if the retailer starts selling the pills.

The religious coalition behind the petition owns about $172 million in shares of the five companies. The coalition was led by Boise, Idaho-based Inspire Investing, which manages $3 billion of assets, and includes the investment arm of the Southern Baptist churches and the American Family Association, a Christian fundamentalist group.