Obama landslides in November (he wins North Dakota, Nebraska, Indiana, and both Carolinas), since it turns out that the allegations from the 2000 campaign that McCain had fathered a black child were in fact true after all.
Other possibilities:
- McCain and Obama decide to run together under a joint Democratic-Republican/Unity '08 ticket (McCain as presidential candidate, Obama as his running mate) and they take all 50 states.
- Obama narrowly wins the election and appoints his father Secretary of State after the election.
P.S. What if Obama were in fact McCain's father? (have to bring a time travel element in here)