I have a really hard time seeing Flake win if Sinema is the nominee and she runs a decent campaign and tries to ignore her religion. I can see it being used against her in ads in the more conservative part of the state.
Even if they do that, it still would have a good chance of backfiring. Arizona is more secular and socially progressive (ie--libertarianish) than the nation at large, and many of those conservatives in those areas are themselves non-religious, pro-choice, and don't care about things like the LGBT culture wars. Take Kelli Ward for example: she's crazy, sure, but she's actually fairly moderate (read: libertarian) on social issues: she's nominally pro-choice and from what I can tell, never even brings up religion and other non-immigration social issues. So, I do think it would be pretty idiotic and tone-deaf for any outside groups to make Sinema's life style an issue.
Didn't they do that for her congressional campaign?