A few reasons.
First, it's becoming a swing state, and Trump carried it but not very decisively. He won by less than 4 points and failed to reach 50 percent. (Romney and McCain won it by more like 9 points.) No Republican is going to win Arizona as easily as they used to.
Second, Sinema isn't running as a whacko. She's running as a moderate, and the media is helping her do it. (See Beto O'Rourke.) Furthermore, she's at least superficially a good candidate. She's reasonably well-spoken and intelligent, and sometimes she's hot.
Third, a lot of Republicans in Arizona are dumbasses who tried to "pull a Christine O'Donnell" and throw away the seat. McSally had to piss away a lot of time, effort, and money to defeat two horrifically bad candidates in the primary, and that always damages the eventual winner. Sinema only had token opposition, whom she effortlessly defeated.
What's sad is that McSally is a better candidate than Sinema is. Without the primary challenge, she wouldn't rout Sinema, but she'd probably win reasonably comfortably. Instead, it's a nail-biter that the GOP could very easily lose.
Last edited: Oct 12, 2018