Wal-Mart Stores Inc. says it has withdrawn its membership in a conservative political group, saying it has strayed from its core mission to advance free market principles.

The group, the American Legislative Exchange Council, known as ALEC, has become a lightning rod for political debate in recent months, especially its previous support for the "Stand Your Ground" self-defense laws at issue in the Florida case in which teenager Trayvon Martin was killed.

Wal-Mart has been a member since 1993.

Wal-Mart's decision follows 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.

ALEC has said that it is being unfairly targeted as part of a broader campaign against its agenda.

Wal-Mart's annual shareholder meeting is Friday.