wedge issue

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A sharply divisive political issue, especially one that is raised by a candidate or party in hopes of attracting or disaffecting a portion of an opponent's customary supporters.

Wiktionary

1. n. politics A discussion point or a difference in points of view which is particularly likely to be controversial and a source of discord between discussants or other involved parties; a difference in viewpoints which causes disharmony within a political or social group.