Here are some thoughts about Ann Kirkpatrick's to challenge McCain:
First, the big winner is Kyrsten Sinema. I know that's counter intuitive, but in a race against McCain, Kirkpatrick and Sinema would both struggle for name ID and media coverage. Just remember how much Fred Duval struggled to get coverage during the Gubernatorial Election. But if they run against each other, they will have a really interesting, high profile primary that will be worth millions to the ultimate winner. So if Sinema was considering running, the odds just moved more in her favor.
Second, I don't think this is a sign of a thin bench. Yes, the Dems have a thin bench and while some of that is due to Napolitano abandoning the Arizona Democratic Party, much of it is also do to the fact that the Dems have been wiped out nationally. However, Kirkpatrick is a sitting multi-term Congresswoman and can't be described as a weak candidate. She may get crushed, but that's because McCain is a very strong candidate.
If you want an indication of how thin the Democratic Bench is, look towards the candidates who try to replace her...someone from the Flagstaff City Council is probably the Dem's best shot. Now THATS a thin bench.
Third. Kirkpatrick's run against McCain is a long shot, so it would seem that she isn't too optimistic about retaining her seat in 2016. She may be looking at the redistricting case and see that she's likely to be in a less favorable district, but I think it's more likely that she realizes that even in the same district and even in a Presidential election year, the Democratic brand is so tarnished that she would have trouble retaining her seat. So why not go out in a blaze of glory?
My prediction is that she loses to McCain and is replaced by a Republican--but that's not a very bold prediction.