Obama would've come very close to carrying Arizona in 2008 if [John] McCain wasn't the nominee.
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I don't see how Arizona stays so Republican in the future. It's very likely going to have a relative shift like we've seen in other southwest states (CA, NV, CO, NM).
It won't.
Arizona was one of seven states that did not shift at least 2.47 percent points more Democratic in 2008. Why that number,
2.47? Because George W. Bush won the popular vote in 2004 by a margin of 2.46, and a shift of 2.47 in however many states was indicative of Barack Obama, the Democratic pickup presidential winner, being able to win over the U.S. Popular Vote. (Turns out there 43 states that shifted to at least the minimum level.)
A 2008 Arizona did not shift as far because it was native son John McCain nominated at the top of the Republican ticket.
Had Barack Obama won re-election in 2012 by more traditional numbers (say, three to five percentage points in his popular-vote margin and an increase with his electoral-vote score), he would won a Democratic pickup of Arizona (as well as Georgia). It would have theoretically been Mitt Romney winning, as he did, Republican pickups from Indiana and Nebraska's 2nd Congressional District (similar-level margins) while Obama countered with Democratic pickups with Georgia and Arizona, for 374 electoral votes.
In every presidential election since after the 1980s, the Republicans have averaged between 7 and 9 electoral votes per state carried. The Democrats have averaged between 11 and 13 electoral votes per state carried. Those 16 from Georgia and 11 from Arizona fall right in line with that average.