House impeaches Obama over use of force in Libya. (Two counts: 1. Violation of War Powers Act by not briefing Congress after the 60 day deadline. 2. Lying to Congress about continuing operations.) Senate refuses to hear the case. Senate Republican leaders criticize their House counterparts. Some House Democrats who voted against action break away from the democratic Party.
Potential results include:
Pro-Intervention Republican third party challenge (Libertarian, Constitution, or Independent)
Anti-Intervention Democrat third party challenge (Green, Peace and Freedom, or Independent)
Unity Anti-Intervention ticket
More than one of the above.
Alternate Scenario 1:
The Duelfer and Kay reports document many of America's intelligence failures in the war with Iraq, plus Iraq's lack of WMD. War opponents point out that not only did Bush support the war, ignoring the intelligence saying otherwise, but John Kerry did as well. As a result, third party candidates gain traction and support. After a close race, third party supporters vow to fight on. Several victories are won by third party congressional candidates (including the high profile defeat of Nancy Pelosi by Cindy Sheehan), and several candidates switching parties (Notably Ron Paul, though Cynthia McKinney lost her reelection bid in 2006 after switching to the Greens). Among the 2012 choices include the Libertarian Paul/Johnson ticket, the Green Kucinich/Stein ticket, the Constitution ticket of Jones/Goode and the Peace and Freedom coalition of Sheehan/McKinney.
Alternate scenario 2:
President Gore somehow passes public financing of elections. Third parties are major beneficiaries.