While a growing number of public media polls have shown Clinton leading or tying Trump in the state of Arizona, I presume the Clinton campaign is basing its latest actions on their own internal polls.
In any event, the Clinton campaign is directing significant resources to win the state.
[Clinton campaign manager Roby Mook] announced that the campaign would be adding $2 million in new spending in Arizona on TV ads, digital ads, and direct mail. It is also dispatching Sen. Bernie Sanders, Chelsea Clinton, and Michelle Obama to campaign in the state this next week. Mook said Clinton herself might start stumping in the state in the final three weeks before Election Day.
I don't know if she will win Arizona, but they're pulling out the big guns for it. Michelle Obama is easily Clinton's best surrogate.
Clinton obviously doesn't need to win Arizona to win the presidential race, but doing so would be a knockout blow and could permanently transform the electorate by solidifying the southwest for Democrats. Competing in Arizona may also boost Democratic Senate candidate Ann Kirkpatrick over John McCain.