Marc Ambinder considers Obama's venture into McCain's backyard, and gives us a heads-up on the leading indicators:
Watch Pima County, the second largest after Maricopa. Obama is likely to win the county by a large margin. If he's going to win the state, and assuming that McCain takes Maricopa by five points or so, Obama will need a margin that is about 20 points over McCain, a tall task, but doable if turnout is very high. As of '06, Maricopa had 3.8 million people living in it; Pima had just under a million. (The margins cancel if turnout rates are equivalent in the two counties. )
Of course, it might be a long night in Pima County... local journalists expect delays and vote counting issues.