McSally would be smart to hammer Sinema on her more progressive past. If she successfully does that, Sinema's entire campaign is essentially defused. She gets neither the progressive base (given her obnoxiously over the top centrist rhetoric) nor the "moderates" (given her past).
I'm skeptical of the notion that the progressive base is not going to show up in a Trump midterm just because their candidate is running to the center. They will complain but they'll show up at the end of the day because there's still a big difference between Sinema and McSally, and people who are tuned into politics know this more than anyone.
McSally has been playing up her military background, but this is one of those things that doesn't affect most of the electorate directly. Her votes to cut Medicaid and repeal Obamacare could become more important than her military background... if Sinema emphasizes those issues. So -- I'd say she's still vulnerable.