"We knew going up against Wal-Mart would raise some eyebrows - and it did," Mullin writes in the letter. "But, simply put, we could not condone the decision of one retailer, even the largest retailer in the world, if it would come at the expense of everyone else. So, we spoke up."

Mullin goes on to say that it is imperative that businesses, associations and politicians "not shy away from deal-breakers like employer mandates."

See the letter here.

Wal-Mart is not a member of the NRF, which traditionally has represented department store companies like Macy's and JCPenney's.

On June 30, Wal-Mart, along with the Service Employees International Union and the Center for American Progress, sent a letter to President Obama supporting the employer mandate, requiring businesses to provide health care coverage to their employees. The move, which marked an unlikely alliance between the retail giant and two liberal groups surprised much of Washington's business community lobby which has been unanimously opposed to an employer mandate.

The National Retail Federation and the Retail Industry Leaders Association (of which Wal-Mart is a member) called off their merger plans, just six days before that letter to the White House. Mullin had planned to retire after the merger, but has since put off those plans.