wedge issue

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun 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.