NEW YORK — Wal-Mart Stores Inc. has withdrawn its membership in a conservative political group that has come under fire as part of the Trayvon Martin case, saying it has strayed from its core mission to advance free-market principles.

The group, the American Legislative Exchange Council, or ALEC, is made up of a group of lawmakers and private sector officials and has become a lightning rod for political debate in recent months. Wal-Mart has been a member since 1993.

Liberal activists complain that ALEC, which has been around for nearly 40 years, improperly influences legislation around the country by allowing corporate leaders to develop model legislation alongside state lawmakers. They also point to some of the laws that ALEC has helped spread around the country, such as ID rules for voters and the so-called “Stand Your Ground” self-defense law that’s at the center of the Florida case in which teenager Martin was killed.

Wal-Mart’s decision follows several other companies’ withdrawal from the group, including Amazon.com, Coca-Cola Co., Kraft Foods Inc., McDonald’s Corp., PepsiCo Inc. and the Bill & Melinda Gates Foundation.